See bucky bits.
2. The "clover" or "Command" key on a Macintosh; use of this term usually reveals that the speaker hacked PCs before coming to the Mac (see also feature key). Some Mac hackers, confusingly, reserve "alt" for the Option key (and it is so labelled on some Mac II keyboards).
3. (Obsolete PDP-10; often "ALT") An alternate name for the ASCII ESC character (Escape, ASCII 27), after the keycap labelling on some older terminals; also "altmode" (/awlt'mohd/). This character was almost never pronounced "escape" on an ITS system, in TECO or under TOPS-10, always alt, as in "Type alt alt to end a TECO command" or "alt-U onto the system" (for "log onto the [ITS] system"). This usage probably arose because alt is easier to say.
4. <messaging> One of the Usenet newsgroup hierarchies. It was founded by John Gilmore and Brian Reid. The alt hierarchy is special in that anyone can create new groups here without going though the normal voting proceduers, hence the regular appearence of new groups with names such as "alt.swedish.chef.bork.bork.bork".