1. <database, information science> A user's (or {agent}'s)
	request for information, generally as a formal request to a
	{database} or {search engine}.

	{SQL} is the most common {database query language}.

	2. <character> {question mark}.

	(1997-04-09)

<programming> In {subject-oriented programming}, a subject is
	a collection of {classes} or class fragments whose {class
	hierarchy} models its domain in its own, subjective way.  A
	subject may be a complete application in itself, or it may be
	an incomplete fragment that must be composed with other
	subjects to produce a complete application.  Subject
	composition combines class hierarchies to produce new subjects
	that incorporate functionality from existing subjects.

	(1999-08-31)

<programming> Any piece of program code in a {high-level
	language} which, when (if) its execution terminates, returns a
	value.  In most programming languages, expressions consist of
	constants, variables, operators, functions, and {parentheses}.
	The operators and functions may be built-in or user defined.
	Languages differ on how expressions of different {types} may
	be combined - with some combination of explicit {casts} and
	implicit {coercions}.

	The {syntax} of expressions generally follows conventional
	mathematical notation, though some languages such as {Lisp} or
	{Forth} have their own idiosyncratic syntax.

	(2001-05-14)

<operating system> A technique for increasing the memory space
	available by moving infrequently-used parts of a program's
	{working memory} from {RAM} to a secondary storage medium,
	usually {hard {disk}.  The unit of transfer is called a page.

	A {memory management unit} (MMU) monitors accesses to memory
	and splits each address into a page number (the most
	significant bits) and an offset within that page (the lower
	bits).  It then looks up the page number in its page table.
	The page may be marked as paged in or paged out.  If it is
	paged in then the memory access can proceed after translating
	the {virtual address} to a {physical address}.  If the
	requested page is paged out then space must be made for it by
	paging out some other page, i.e. copying it to disk.  The
	requested page is then located on the area of the disk
	allocated for "{swap space}" and is read back into {RAM}.  The
	page table is updated to indicate that the page is paged in
	and its physical address recorded.

	The MMU also records whether a page has been modified since it
	was last paged in.  If it has not been modified then there is
	no need to copy it back to disk and the space can be reused
	immediately.

	Paging allows the total memory requirements of all running
	tasks (possibly just one) to exceed the amount of {physical
	memory}, whereas {swapping} simply allows multiple processes
	to run concurrently, so long as each process on its own fits
	within {physical memory}.

	(1996-11-22)

<data, data processing, jargon> /day't*/ (Or "raw data")
	Numbers, {characters}, {images}, or other method of recording,
	in a form which can be assessed by a human or (especially)
	input into a {computer}, stored and {processed} there, or
	transmitted on some {digital channel}.  Computers nearly
	always represent data in {binary}.

	Data on its own has no meaning, only when interpreted by some
	kind of {data processing system} does it take on meaning and
	become {information}.

	People or computers can find patterns in data to perceive
	information, and information can be used to enhance
	{knowledge}.  Since knowledge is prerequisite to wisdom, we
	always want more data and information.  But, as modern
	societies verge on {information overload}, we especially need
	better ways to find patterns.

	1234567.89 is data.

	"Your bank balance has jumped 8087% to $1234567.89" is
	information.

	"Nobody owes me that much money" is knowledge.

	"I'd better talk to the bank before I spend it, because of
	what has happened to other people" is wisdom.

	(1999-04-30)

1. <mathematics> (Or "map", "mapping") If D and C are sets
	(the domain and codomain) then a function f from D to C,
	normally written "f : D -> C" is a subset of D x C such that:

	1. For each d in D there exists some c in C such that (d,c) is
	an element of f.  I.e. the function is defined for every
	element of D.

	2. For each d in D, c1 and c2 in C, if both (d,c1) and (d,c2)
	are elements of f then c1 = c2.  I.e. the function is uniquely
	defined for every element of D.

	See also {image}, {inverse}, {partial function}.

	2. <programming> Computing usage derives from the mathematical
	term but is much less strict.  In programming (except in
	{functional programming}), a function may return different
	values each time it is called with the same argument values
	and may have {side effects}.

	A {procedure} is a function which returns no value but has
	only {side-effects}.  The {C} language, for example, has no
	procedures, only functions.  {ANSI C} even defines a {type},
	{void}, for the result of a function that has no result.

	(1996-09-01)