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}, or (rarely) {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.


Source: The Jargon File