1. Not operating.  "The up escalator is down" is considered a
	humorous thing to say, and "The elevator is down" always
	means "The elevator isn't working" and never refers to what
	floor the elevator is on.  With respect to computers, this
	term has passed into the mainstream; the extension to other
	kinds of machine is still hackish.

	2. "go down" To stop functioning; usually said of the
	{system}.  The message from the {console} that every hacker
	hates to hear from the operator is "System going down in 5
	minutes".

	3. "take down", "bring down" To deactivate purposely, usually
	for repair work or {PM}.  "I'm taking the system down to work
	on that bug in the tape drive."  Occasionally one hears the
	word "down" by itself used as a verb in this sense.

	See {crash}; opposite: {up}.

	[{Jargon File}]

	(1994-12-07)

<messaging> (e-mail) Messages automatically passed from one
	computer user to another, often through computer {networks}
	and/or via {modems} over telephone lines.

	A message, especially one following the common {RFC 822}
	{standard}, begins with several lines of {headers}, followed
	by a blank line, and the body of the message.  Most e-mail
	systems now support the {MIME} {standard} which allows the
	message body to contain "{attachments}" of different kinds
	rather than just one block of plain {ASCII} text.  It is
	conventional for the body to end with a {signature}.

	Headers give the name and {electronic mail address} of the
	sender and recipient(s), the time and date when it was sent
	and a subject.  There are many other headers which may get
	added by different {message handling systems} during delivery.

	The message is "composed" by the sender, usually using a
	special program - a "{Mail User Agent}" (MUA).  It is then
	passed to some kind of "{Message Transfer Agent}" (MTA) - a
	program which is responsible for either delivering the message
	locally or passing it to another MTA, often on another {host}.
	MTAs on different hosts on a network often communicate using
	{SMTP}.  The message is eventually delivered to the
	recipient's {mailbox} - normally a file on his computer - from
	where he can read it using a mail reading program (which may
	or may not be the same {MUA} as used by the sender).

	Contrast {snail-mail}, {paper-net}, {voice-net}.

	The form "email" is also common, but is less suggestive of the
	correct pronunciation and derivation than "e-mail".  The word
	is used as a noun for the concept ("Isn't e-mail great?", "Are
	you on e-mail?"), a collection of (unread) messages ("I spent
	all night reading my e-mail"), and as a verb meaning "to send
	(something in) an e-mail message" ("I'll e-mail you (my
	report)").  The use of "an e-mail" as a count noun for an
	e-mail message, and plural "e-mails", is now (2000) also well
	established despite the fact that "mail" is definitely a mass
	noun.

	Oddly enough, the word "emailed" is actually listed in the
	Oxford English Dictionary.  It means "embossed (with a raised
	pattern) or arranged in a net work".  A use from 1480 is
	given.  The word is derived from French "emmailleure",
	network.  Also, "email" is German for enamel.

	{The story of the first e-mail message
	(http://www.pretext.com/mar98/features/story2.htm)}.

	(2002-07-14)

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)