Many of the old-school D&D acronyms began life on the DragonsFoot Forums, since it's the largest online community of folks into the pre-Wizards of the Coast versions of D&D.
Since many folks actually give a darn (whether it matters or no) about the slight (depending on your perspective) differences between various iterations of D&D (particularly between the different flavors of so-called 'Basic', or rather, as we say "Classic" and Original D&D), many of these acronyms have become common between folks who communicate in forums and blogs concerning old-school D&D in general.
Thus, for future reference:
OD&D - Original D&D (1974) - Note that this can refer just and only to the original box, with only the Three Little Brown Booklets (or, 3LBBs), but can also refer to the game as 'whole'. That is to say, some or all the following supplements (Supplement I: Greyhawk, Supplement II: Blackmoor, etc.)
Classic: Otherwise known as the "Basic D&D" game. This is a 'catch-all' phrase for everything from the Holmes Basic set (or "Blue Book", from '77(to '79)...even though that's a once-over of an intro the OD&D (thought the game's interior texts says otherwise) *confused yet?!?
* ...through Moldvay Basic and Cook/Marsh Expert (the aforementioned B/X), on to the Mentzer-edited versions of the line in 1983 (Basic, Expert, Companion, Masters, Immortals), known simply as "Mentzer" or "BECMI" (note that every once in a while, some folks
will differentiate between Basic and Expert by Moldvay/Cook and Basic and Expert by Mentzer, in which case it's "B/X" and "B/E"...usually...
This category also includes the Rules Cyclopedia (the RC), being a compiled version of BECM (though very little to no "I"), as well as the mid to late 90's intro boxes for RC, the NEW Easy To Master D&D Game (the "Black Box") as well as The Classic Dungeons & Dragons Game boxes.
The rest is pretty straightforward.
AD&D1E - Advanced D&D first edition, it's supplementary material is usually easily figured out...that is, the Dungeoneer's Survival Guide is sometimes simply noted as the DSG, and so on.
2E - Second Edition
etc.
etc.
Blah diddy blah
...hhmm....
I just made it worse and not better didn't I?