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.