The Jargon File (version 4.4.7, 29 Dec 2003):

alt bit
 /awlt bit/, adj.

    See meta bit.


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

meta bit
alt bit

    The most-signigicant bit of an 8-bit character,
   which is on in character values 128--255.  Also called the high
   bit, "alt bit" (alternate bit) or hobbit.  Some terminals and
   consoles (see space-cadet keyboard) have a META shift key.
   Others (including, *mirabile dictu*, keyboards on IBM PC-class
   machines) have an ALT key.  See also bucky bits.

   Historical note: although, in modern usage shaped by a universe of
   8-bit bytes, the meta bit is invariably hex 80 (octal 0200),
   things were different on earlier machines with 36-bit words and
   9-bit bytes.  The MIT and Stanford keyboards (see space-cadet
   keyboard) generated hex 100 (octal 400) from their meta keys.

   [Jargon File]

   (2014-10-05)