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.