Alt

from The Collaborative International Dictionary of English v.0.48

Alt \Alt\, a. & n. [See {Alto}.] (Mus.)
   The higher part of the scale. See {Alto}.
   [1913 Webster]

   {To be in alt}, to be in an exalted state of mind.
      [1913 Webster] Altaian

from Jargon File (4.4.4, 14 Aug 2003)

alt
 /awlt/

   1. n. The alt shift key on an IBM PC or {clone} keyboard; see {bucky
   bits}, sense 2 (though typical PC usage does not simply set the 0200
   bit).

   2. n. The option key on a Macintosh; use of this term usually reveals
   that the speaker hacked PCs before coming to the Mac (see also
   {feature key}, which is sometimes incorrectly called `alt').

   3. The alt hierarchy on Usenet, the tree of newsgroups created by
   users without a formal vote and approval procedure. There is a myth,
   not entirely implausible, that alt is acronymic for "anarchists,
   lunatics, and terrorists"; but in fact it is simply short for
   "alternative".

   4. n.,obs. Rare alternate name for the ASCII ESC character (ASCII
   0011011). This use, derives, with the alt key itself, from archaic
   PDP-10 operating systems, especially {ITS}.

from The Free On-line Dictionary of Computing (8 July 2008)

alt
altmode

   <character> /awlt/ 1. The alt {modifier key} on many
   {keyboards}, including the {IBM PC}.  On some keyboards and
   {operating systems}, (but not the IBM PC) the alt key sets bit
   7 of the character generated.

   See {bucky bits}.

   2. The "{clover}" or "Command" key on a {Macintosh}; use of
   this term usually reveals that the speaker hacked PCs before
   coming to the Mac (see also {feature key}).  Some Mac hackers,
   confusingly, reserve "alt" for the Option key (and it is so
   labelled on some Mac II keyboards).

   3. (Obsolete {PDP-10}; often "ALT") An alternate name for the
   {ASCII} ESC character (Escape, ASCII 27), after the keycap
   labelling on some older {terminals}; also "altmode"
   (/awlt'mohd/).  This character was almost never pronounced
   "escape" on an {ITS} system, in {TECO} or under {TOPS-10},
   always alt, as in "Type alt alt to end a TECO command" or
   "alt-U onto the system" (for "log onto the [ITS] system").
   This usage probably arose because alt is easier to say.

   4. <messaging> One of the {Usenet} {newsgroup} {hierarchies}.
   It was founded by {John Gilmore} and {Brian Reid}.  The alt
   hierarchy is special in that anyone can create new groups here
   without going though the normal voting proceduers, hence the
   regular appearence of new groups with names such as
   "alt.swedish.chef.bork.bork.bork".

   [{Jargon File}]

   (1997-04-12)