FreeBSD Glossary

Table of Contents

This glossary contains terms and acronyms used within the FreeBSD community and documentation.


ACPI Machine Language

Pseudocode, interpreted by a virtual machine within an ACPI-compliant operating system, providing a layer between the underlying hardware and the documented interface presented to the OS.

ACPI Source Language

The programming language AML is written in.

Access Control List

A list of permissions attached to an object, usually either a file or a network device.

Advanced Configuration and Power Interface

A specification which provides an abstraction of the interface the hardware presents to the operating system, so that the operating system should need to know nothing about the underlying hardware to make the most of it. ACPI evolves and supersedes the functionality provided previously by APM, PNPBIOS and other technologies, and provides facilities for controlling power consumption, machine suspension, device enabling and disabling, etc.

Application Programming Interface

A set of procedures, protocols and tools that specify the canonical interaction of one or more program parts; how, when and why they do work together, and what data they share or operate on.

Advanced Power Management

An API enabling the operating system to work in conjunction with the BIOS in order to achieve power management. APM has been superseded by the much more generic and powerful ACPI specification for most applications.

Advanced Programmable Interrupt Controller

Advanced Technology Attachment

Asynchronous Transfer Mode

Authenticated Post Office Protocol

Automatic Mount Daemon

A daemon that automatically mounts a filesystem when a file or directory within that filesystem is accessed.


Base Address Register

The registers that determine which address range a PCI device will respond to.

Basic Input/Output System

The definition of BIOS depends a bit on the context. Some people refer to it as the ROM chip with a basic set of routines to provide an interface between software and hardware. Others refer to it as the set of routines contained in the chip that help in bootstrapping the system. Some might also refer to it as the screen used to configure the bootstrapping process. The BIOS is PC-specific but other systems have something similar.

Berkeley Internet Name Domain

An implementation of the DNS protocols.

Berkeley Software Distribution

This is the name that the Computer Systems Research Group (CSRG) at The University of California at Berkeley gave to their improvements and modifications to AT&T’s 32V UNIX®. FreeBSD is a descendant of the CSRG work.

Bikeshed Building

A phenomenon whereby many people will give an opinion on an uncomplicated topic, whilst a complex topic receives little or no discussion. See the FAQ for the origin of the term.


Carrier Detect

An RS232C signal indicating that a carrier has been detected.

Central Processing Unit

Also known as the processor. This is the brain of the computer where all calculations take place. There are a number of different architectures with different instruction sets. Among the more well-known are the Intel-x86 and derivatives, Arm, and PowerPC.

Challenge Handshake Authentication Protocol

A method of authenticating a user, based on a secret shared between client and server.

Classical IP over ATM

Clear To Send

An RS232C signal giving the remote system permission to send data.

Common Object File Format


Discretionary Access Control

Data Encryption Standard

A method of encrypting information, traditionally used as the method of encryption for UNIX® passwords and the crypt(3) function.

Data Set Ready

An RS232C signal sent from the modem to the computer or terminal indicating a readiness to send and receive data.

Data Terminal Ready

An RS232C signal sent from the computer or terminal to the modem indicating a readiness to send and receive data.


An interactive in-kernel facility for examining the status of a system, often used after a system has crashed to establish the events surrounding the failure.

Differentiated System Description Table

An ACPI table, supplying basic configuration information about the base system.

Distance-Vector Multicast Routing Protocol

Domain Name System

The system that converts humanly readable hostnames (i.e., to Internet addresses and vice versa.

Dynamic Host Configuration Protocol

A protocol that dynamically assigns IP addresses to a computer (host) when it requests one from the server. The address assignment is called a “lease”.



See Extended COFF.


See Executable and Linking Format.


See Encapsulated Security Payload.

Encapsulated Security Payload

Executable and Linking Format

Extended COFF


File Allocation Table

File Allocation Table (16-bit)

File Transfer Protocol

A member of the family of high-level protocols implemented on top of TCP which can be used to transfer files over a TCP/IP network.

Fixed ACPI Description Table



The name of a mutual exclusion mechanism (a sleep mutex) that protects a large set of kernel resources. Although a simple locking mechanism was adequate in the days where a machine might have only a few dozen processes, one networking card, and certainly only one processor, in current times it is an unacceptable performance bottleneck. FreeBSD developers are actively working to replace it with locks that protect individual resources, which will allow a much greater degree of parallelism for both single-processor and multi-processor machines.

Graphical User Interface

A system where the user and computer interact with graphics.



HyperText Markup Language

The markup language used to create web pages.


IP Firewall

IP Version 4

The IP protocol version 4, which uses 32 bits for addressing. This version is still the most widely used, but it is slowly being replaced with IPv6.

IP Version 6

The new IP protocol. Invented because the address space in IPv4 is running out. Uses 128 bits for addressing.


Intel’s ASL compiler

Intel’s compiler for converting ASL into AML.

Internet Message Access Protocol

A protocol for accessing email messages on a mail server, characterised by the messages usually being kept on the server as opposed to being downloaded to the mail reader client.

See Also Post Office Protocol Version 3.

Internet Printing Protocol

Internet Protocol

The packet transmitting protocol that is the basic protocol on the Internet. Originally developed at the U.S. Department of Defense and an extremely important part of the TCP/IP stack. Without the Internet Protocol, the Internet would not have become what it is today. For more information, see RFC 791.

Internet Service Provider

A company that provides access to the Internet.



Japanese for “turtle”, the term KAME is used in computing circles to refer to the KAME Project, who work on an implementation of IPv6.


See Key Distribution Center.


See Kernel ld(1).


See Kernel Scheduler Entities.


See Kernel Virtual Address.


See Kilo Bits Per Second.

Kernel ld(1)

A method of dynamically loading functionality into a FreeBSD kernel without rebooting the system.

Kernel Scheduler Entities

A kernel-supported threading system. See the project home page for further details.

Kernel Virtual Address

Key Distribution Center

Kilo Bits Per Second

Used to measure bandwidth (how much data can pass a given point at a specified amount of time). Alternates to the Kilo prefix include Mega, Giga, Tera, and so forth.


Line Printer Daemon

Local Area Network

A network used on a local area, e.g. office, home, or so forth.

Lock Order Reversal

The FreeBSD kernel uses a number of resource locks to arbitrate contention for those resources. A run-time lock diagnostic system found in FreeBSD-CURRENT kernels (but removed for releases), called witness(4), detects the potential for deadlocks due to locking errors. (witness(4) is actually slightly conservative, so it is possible to get false positives.) A true positive report indicates that “if you were unlucky, a deadlock would have happened here”.

True positive LORs tend to get fixed quickly, so check and the LORs Seen page before posting to the mailing lists.


Mail Transfer Agent

An application used to transfer email. An MTA has traditionally been part of the BSD base system. Today Sendmail is included in the base system, but there are many other MTAs, such as postfix, qmail and Exim.

Mail User Agent

An application used by users to display and write email.

Mandatory Access Control

Massachusetts Institute of Technology

Merge From Current

To merge functionality or a patch from the -CURRENT branch to another, most often -STABLE.

Merge From Head

To merge functionality or a patch from a repository HEAD to an earlier branch.

Merge From Stable

In the normal course of FreeBSD development, a change will be committed to the -CURRENT branch for testing before being merged to -STABLE. On rare occasions, a change will go into -STABLE first and then be merged to -CURRENT.

This term is also used when a patch is merged from -STABLE to a security branch.

Merge From Vendor

Message Of The Day

A message, usually shown on login, often used to distribute information to users of the system.

Multi-Level Security

Multiple APIC Description Table


Network Address Translation

A technique where IP packets are rewritten on the way through a gateway, enabling many machines behind the gateway to effectively share a single IP address.

Network File System

New Technology File System

A filesystem developed by Microsoft and available in its “New Technology” operating systems, such as Windows® 2000, Windows NT® and Windows® XP.

Network Time Protocol

A means of synchronizing clocks over a network.


On-Demand Mail Relay

Operating System

A set of programs, libraries and tools that provide access to the hardware resources of a computer. Operating systems range today from simplistic designs that support only one program running at a time, accessing only one device to fully multi-user, multi-tasking and multi-process systems that can serve thousands of users simultaneously, each of them running dozens of different applications.

Overtaken By Events

Indicates a suggested change (such as a Problem Report or a feature request) which is no longer relevant or applicable due to such things as later changes to FreeBSD, changes in networking standards, the affected hardware having since become obsolete, and so forth.


PPP over ATM

PPP over Ethernet


See Problem Report.


See Preboot eXecution Environment.

Password Authentication Protocol

Personal Computer

Personal Computer Network File System Daemon

Physical Address Extensions

A method of enabling access to up to 64 GB of RAM on systems which only physically have a 32-bit wide address space (and would therefore be limited to 4 GB without PAE).

Pluggable Authentication Modules

Point-to-Point Protocol

Pointy Hat

A mythical piece of headgear, much like a dunce cap, awarded to any FreeBSD committer who breaks the build, makes revision numbers go backwards, or creates any other kind of havoc in the source base. Any committer worth his or her salt will soon accumulate a large collection. The usage is (almost always?) humorous.

Portable Document Format

Post Office Protocol

See Also Post Office Protocol Version 3.

Post Office Protocol Version 3

A protocol for accessing email messages on a mail server, characterised by the messages usually being downloaded from the server to the client, as opposed to remaining on the server.

PostScript Printer Description

Preboot eXecution Environment

Principle Of Least Astonishment

As FreeBSD evolves, changes visible to the user should be kept as unsurprising as possible. For example, arbitrarily rearranging system startup variables in /etc/defaults/rc.conf violates POLA. Developers consider POLA when contemplating user-visible system changes.

Problem Report

A description of some kind of problem that has been found in either the FreeBSD source or documentation. See Writing FreeBSD Problem Reports.

Process ID

A number, unique to a particular process on a system, which identifies it and allows actions to be taken against it.

Project Evil

The working title for the NDISulator, written by Bill Paul, who named it referring to how awful it is (from a philosophical standpoint) to need to have something like this in the first place. The NDISulator is a special compatibility module to allow Microsoft Windows™ NDIS miniport network drivers to be used with FreeBSD/i386. This is usually the only way to use cards where the driver is closed-source. See src/sys/compat/ndis/subr_ndis.c.


Random Access Memory

Revision Control System

The Revision Control System (RCS) is one of the oldest software suites that implement “revision control” for plain files. It allows the storage, retrieval, archival, logging, identification and merging of multiple revisions for each file. RCS consists of many small tools that work together. It lacks some of the features found in more modern revision control systems, like Git, but it is very simple to install, configure, and start using for a small set of files.

Received Data

An RS232C pin or wire that data is received on.

Recommended Standard 232C

A standard for communications between serial devices.

Reduced Instruction Set Computer

An approach to processor design where the operations the hardware can perform are simplified but made as general purpose as possible. This can lead to lower power consumption, fewer transistors and in some cases, better performance and increased code density. Examples of RISC processors include the Alpha, SPARC®, ARM® and PowerPC®.

Redundant Array of Inexpensive Disks

Remote Procedure Call

Request For Comments

A set of documents defining Internet standards, protocols, and so forth. See

Also used as a general term when someone has a suggested change and wants feedback.

Request To Send

An RS232C signal requesting that the remote system commences transmission of data.

Router Advertisement


SMTP Authentication

Server Message Block

Signal Ground

An RS232 pin or wire that is the ground reference for the signal.

Simple Mail Transfer Protocol

Secure Shell

Small Computer System Interface


Subversion is a version control system currently used by the FreeBSD project.

Suspend To RAM

Symmetric MultiProcessor

System Control Interrupt


Ticket-Granting Ticket

Time Stamp Counter

A profiling counter internal to modern Pentium® processors that counts core frequency clock ticks.

Transmission Control Protocol

A protocol that sits on top of (e.g.) the IP protocol and guarantees that packets are delivered in a reliable, ordered, fashion.

Transmission Control Protocol/Internet Protocol

The term for the combination of the TCP protocol running over the IP protocol. Much of the Internet runs over TCP/IP.

Transmitted Data

An RS232C pin or wire that data is transmitted on.

Trivial FTP


Uniform Resource Locator

A method of locating a resource, such as a document on the Internet and a means to identify that resource.

Unix File System Version 1

The original UNIX® file system, sometimes called the Berkeley Fast File System.

Unix File System Version 2

An extension to UFS1, introduced in FreeBSD 5-CURRENT. UFS2 adds 64 bit block pointers (breaking the 1T barrier), support for extended file storage and other features.

Universal Serial Bus

A hardware standard used to connect a wide variety of computer peripherals to a universal interface.

User ID

A unique number assigned to each user of a computer, by which the resources and permissions assigned to that user can be identified.

User Datagram Protocol

A simple, unreliable datagram protocol which is used for exchanging data on a TCP/IP network. UDP does not provide error checking and correction like TCP.


Virtual Private Network

A method of using a public telecommunication such as the Internet, to provide remote access to a localized network, such as a corporate LAN.

Last modified on: March 9, 2024 by Danilo G. Baio