alpha particles = A = alt bit

alt /awlt/

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. n.,obs. [PDP-10; often capitalized to ALT] 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 usage probably arose 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). 4. 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".

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.