1. <mathematics> A collection of {nodes} and {edges}.

	See also {connected graph}, {degree}, {directed graph}, {Moore
	bound}, {regular graph}, {tree}.

	2. <graphics> A visual representation of algebraic equations
	or data.

	(1996-09-22)

An error condition that changes the normal {flow of control}
	in a program.  An exception may be generated ("raised") by
	{hardware} or {software}.  Hardware exceptions include
	{reset}, {interrupt} or a signal from a {memory management
	unit}.  Exceptions may be generated by the {arithmetic logic
	unit} or {floating-point unit} for numerical errors such as
	divide by zero, {overflow} or {underflow} or {instruction
	decoding} errors such as privileged, reserved, {trap} or
	undefined instructions.  Software exceptions are even more
	varied and the term could be applied to any kind of error
	checking which alters the normal behaviour of the program.

	(1994-10-31)

<language, programming> (Or "source", or rarely "source
	language") The form in which a computer program is written by
	the programmer.  Source code is written in some formal
	programming language which can be compiled automatically into
	{object code} or {machine code} or executed by an
	{interpreter}.

	(1995-01-05)

1. <database> See {data dictionary}.

	2. <programming> The core of a {CASE} tool, typically a {DBMS}
	where all development documents are stored.

	(1999-04-27)

1. <application> Any specific type of {file} produced or
	edited by a specific {application}; usually capable of being
	printed.  E.g. "Word document", "Photoshop document", etc.

	2. <hypertext> A term used on some systems (e.g. {Intermedia})
	for a {hypertext} {node}.  It is sometimes used for a
	collection of nodes on related topics, possibly stored or
	distributed as one.

	3. <programming> To write {documentation} on a certain piece
	of code.

	(2003-10-25)

<jargon> To transfer data from one computer to another.
	Downloading usually refers to transfer from a larger "host"
	system (especially a {server} or {mainframe}) to a smaller
	"client" system, especially a {microcomputer} or specialised
	peripheral, and "{upload}" usually means from small to large.

	Others hold that, technically, download means "receive" and
	upload means "send", irrespective of the size of the systems
	involved.

	Note that in communications between ground and space,
	space-to-earth transmission is always "down" and the reverse
	"up", regardless of size.  So far the in-space machines have
	invariably been smaller; thus the upload/download distinction
	has been reversed from its usual sense.

	[{Jargon File}]

	(2003-11-04)

1. A point or vertex in a {graph}.

	2. {network node}.

	3. A {hypertext} document.

1. A meta-thing.  Used to stand for a real object in didactic
	examples (especially database tutorials).  Legend has it that
	the original widgets were holders for buggy whips.  "But
	suppose the parts list for a widget has 52 entries..."

	2. [possibly evoking "window gadget"] In graphical user
	interfaces, a combination of a graphic symbol and some program
	code to perform a specific function.  E.g. a scroll-bar or
	button.  Windowing systems usually provide widget libraries
	containing commonly used widgets drawn in a certain style and
	with consistent behaviour.

	[{Jargon File}]

<software> Instructions for a computer in some programming
	language, often {machine language}.  The word "code" is often
	used to distinguish instructions from {data} (e.g. "The code
	is marked 'read-only'") whereas "{software}" is used in
	contrast with "{hardware}" and may consist of more than just
	code.

	(2000-04-08)

1. <networking> {segmentation}.

	2. The process, or result, of splitting a large area of free
	memory (on disk or in main memory) into smaller non-contiguous
	blocks.  This happens after many blocks have been allocated
	and freed.  For example, if there is 3 kilobytes of free space
	and two 1k blocks are allocated and then the first one (at the
	lowest address) is freed, then there will be 2k of free space
	split between the two 1k blocks.  The maximum size block that
	could then be allocated would be 1k, even though there was 2k
	free.  The solution is to "compact" the free space by moving
	the allocated blocks to one end (and thus the free space to
	the other).

	As modern file systems are used and files are deleted and
	created, the total free space becomes split into smaller
	non-contiguous blocks (composed of "{clusters}" or "{sectors}"
	or some other unit of allocation).  Eventually new files being
	created, and old files being extended, cannot be stored each
	in a single contiguous block but become scattered across the
	file system.  This degrades performance as multiple {seek}
	operations are required to access a single fragmented file.

	Defragmenting consolidates each existing file and the free
	space into a continuous group of sectors.  Access speed will
	be improved due to reduced seeking.

	The rate of fragmentation depends on the {algorithm} used to
	allocate space and the number and position of free sectors.  A
	nearly-full file system will fragment more quickly.

	{MSDOS} and {Microsoft Windows} use the simplest algorithm to
	allocate free clusters and so fragmentation occurs quickly.  A
	disk should be defragmented before fragmentation reaches 10%.

	See {garbage collection}.

	(1997-08-29)

1. <programming> The prototype for an {object} in an
	{object-oriented language}; analogous to a {derived type} in a
	{procedural language}.  A class may also be considered to be a
	set of objects which share a common structure and behaviour.
	The structure of a class is determined by the {class
	variables} which represent the {state} of an object of that
	class and the behaviour is given by a set of {methods}
	associated with the class.

	Classes are related in a {class hierarchy}.  One class may be
	a specialisation (a "{subclass}") of another (one of its
	"{superclasses}") or it may be composed of other classes or it
	may use other classes in a {client-server} relationship.  A
	class may be an {abstract class} or a {concrete class}.

	See also {signature}.

	2. <programming> See {type class}.

	3. <networking> One of three types of {Internet addresses}
	distinguished by their most significant bits.

	3. <language> A language developed by the {Andrew Project}.
	It was one of the first attempts to add {object-oriented}
	features to {C}.

	(1995-05-01)

[probably from "main store"] In some varieties of Commonwealth
	hackish, the preferred synonym for {core}.  Thus, "bringing a
	program into store" means not that one is returning
	shrink-wrapped software but that a program is being {swap}ped
	in.

	[{Jargon File}]

1. A discrepancy between a computed, observed, or measured
	value or condition and the true, specified, or theoretically
	correct value or condition.

	2. <programming> A mental mistake made by a programmer that
	may result in a program {fault}.

	3. (verb) What a program does when it stops as result of a
	programming error.

	(2000-03-28)

In {object-oriented programming} sending a message to an
	{object} (to invoke a {method}) is equivalent to calling a
	{procedure} in traditional programming languages, except that
	the actual code executed may only be selected at run time
	depending on the {class} of the object.  Thus, in response to
	the message "drawSelf", the method code invoked would be
	different if the target object were a circle or a square.

	(1995-02-16)

1. <software> An occurrence or happening of significance to a
	task or program, such as the completion of an asynchronous
	input/output operation.  A task may wait for an event or any
	of a set of events or it may (request to) receive asynchronous
	notification (a {signal} or {interrupt}) that the event has
	occurred.

	See also {event-driven}.

	2. <data> A transaction or other activity that affects the
	records in a file.

	(2000-02-09)

1. <programming> A common name for the {integer} data type.
	In {C} for example, it means a (signed) integer of the
	computer's native {word length}.

	2. <networking> The {top-level domain} for international
	organisations.

	(1999-01-26)

<programming> A special value used in several languages to
	represent the thing referred to by an uninitialised pointer.

	<database> A special value that may be stored in some database
	columns to represent an unknown, missing, not applicable, or
	undefined value.  Nulls are treated completely differently
	from ordinary values when evaluating SQL expressions and there
	are several SQL constructs for dealing with nulls.

	(2003-06-17)

<hardware> A {SCSI} device that executes a command from a
	{SCSI initiator} to perform some task.  Typically the target
	is a SCSI peripheral device but the {host adapter} can also be
	a target.

	(1999-02-10)

<programming> (See below for synonyms) A data structure for
	storing items which are to be accessed in last-in first-out
	order.

	The operations on a stack are to create a new stack, to "push"
	a new item onto the top of a stack and to "pop" the top item
	off.  Error conditions are raised by attempts to pop an empty
	stack or to push an item onto a stack which has no room for
	further items (because of its implementation).

	Most processors include support for stacks in their
	{instruction set architectures}.  Perhaps the most common use
	of stacks is to store {subroutine} arguments and return
	addresses.  This is usually supported at the {machine code}
	level either directly by "jump to subroutine" and "return from
	subroutine" instructions or by {auto-increment} and
	auto-decrement {addressing modes}, or both.  These allow a
	contiguous area of memory to be set aside for use as a stack
	and use either a special-purpose {register} or a general
	purpose register, chosen by the user, as a {stack pointer}.

	The use of a stack allows subroutines to be {recursive} since
	each call can have its own calling context, represented by a
	stack frame or {activation record}.  There are many other
	uses.  The programming language {Forth} uses a data stack in
	place of variables when possible.

	Although a stack may be considered an {object} by users,
	implementations of the object and its access details differ.
	For example, a stack may be either ascending (top of stack is
	at highest address) or descending.  It may also be "full" (the
	stack pointer points at the top of stack) or "empty" (the
	stack pointer points just past the top of stack, where the
	next element would be pushed).  The full/empty terminology is
	used in the {Acorn Risc Machine} and possibly elsewhere.

	In a list-based or {functional language}, a stack might be
	implemented as a {linked list} where a new stack is an empty
	list, push adds a new element to the head of the list and pop
	splits the list into its head (the popped element) and tail
	(the stack in its modified form).

	At {MIT}, {pdl} used to be a more common synonym for stack,
	and this may still be true.  {Knuth} ("The Art of Computer
	Programming", second edition, vol. 1, p. 236) says:

	  Many people who realised the importance of stacks and queues
	  independently have given other names to these structures:
	  stacks have been called push-down lists, reversion storages,
	  cellars, dumps, nesting stores, piles, last-in first-out
	  ("LIFO") lists, and even yo-yo lists!

	[{Jargon File}]

	(1995-04-10)

1. <chat> {Unix}'s simple {talk} command and {protocol}.
	write has been largely superseded by {talk} and then {irc}.

	An enhancement, {RWP}, has been proposed.

	2. <tool> A simple {text editor} for {Windows}.

	(1998-04-28)

<data> A data structure holding many values, possibly of
	different types, which is usually accessed sequentially,
	working from the head to the end of the tail - an "ordered
	list".  This contrasts with a (one-dimensional) {array}, any
	element of which can be accessed equally quickly.

	Lists are often stored using a cell and pointer arrangement
	where each value is stored in a cell along with an associated
	pointer to the next cell.  A special pointer, e.g. zero, marks
	the end of the list.  This is known as a (singly) "linked
	list".  A doubly linked list has pointers from each cell to
	both next and previous cells.

	An unordered list is a {set}.

	(1998-11-12)

1. <file format> A single file containing one or (usually)
	more separate files plus information to allow them to be
	extracted (separated) by a suitable program.

	Archives are usually created for software distribution or
	{backup}.  {tar} is a common format for {Unix} archives, and
	{arc} or {PKZIP} for {MS-DOS} and {Microsoft Windows}.

	2. <operating system> To transfer files to slower, cheaper
	media (usually {magnetic tape}) to free the {hard disk} space
	they occupied.  This is now normally done for long-term
	storage but in the 1960s, when disk was much more expensive,
	files were often shuffled regularly between disk and tape.

	3. <networking> {archive site}.

	(1996-12-08)