What is A for? I think in the context of RPGs, obviously A has to be for adventure, it's what keeps bringing us all back to the table week in and week out. Adventure in the tradition of Conan the barbarian and all the other guys from appendix N. So it's adventure, at least a little bit.
A is also for ancient. Ancient fallen empires, whose ruins we explore. Ancient secrets we seek to solve. Ancient gods whose cults we oppose. Ancient treasure troves guarded by Ancient Red Dragons!
A is also for Arthur, the king, whose story always held me rapt. I always loved the Mallory version of Arthur the best, probably because it hews closest to the movie Excalibur. Excalibur and D&D are linked in my mind inextricably due to the fact that I first played D&D within days of seeing that movie in the theater as a kid. Arthurian adventures lead us to the next A word by way of Celtic myth that the Arthur cycle draws upon so much.
So A is for Awen, the Welsh word for poetic inspiration, which I freely associate with the inspiration of the DM and, in my Celtic themed Garnia campaign, Bards. Bards have, in the mythology of my campaign, a spirit of inspiration known as Awen, which grants them all of their supernatural abilities.
A is also for Ashigaru. Ashigaru are the lightly armored troops of feudal Japan. When I run Oriental Adventures (or Ruins and Ronin) they are the types of guys who join the party as hirelings, generally they are lightly armored and carry spears.
That's all for A today because I have had a pretty busy day.