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

meta bit
 n.

    The top bit of an 8-bit character, which is on in character values
    128--255. Also called high bit, alt bit. 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.


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)