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

WordNet (r) 3.0 (2006):

ALT
    n 1: angular distance above the horizon (especially of a
         celestial object) [syn: elevation, EL, altitude,
         ALT]

The Jargon File (version 4.4.7, 29 Dec 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.


The Free On-line Dictionary of Computing (30 December 2018):

alt
altmode

    /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.  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)