1. n. The ALT shift key on an IBM PC or clone.
2. [possibly lowercased] n. 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 command key). Some Mac hackers, confusingly, reserve 'ALT' for the Option key.
3. n.obs. [PDP-10] Alternate name for the ASCII ESC character (ASCII 0011011), after the keycap labeling 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 was probably because ALT is more convenient to say than 'escape', especially when followed by another ALT or a character (or another ALT *and* a character, for that matter).