A is for Altair, a major star system that may or may not have been in the Federation during the time of TOS
B is for Bolius. The musicians there really know how to sing the blues.
C is for Cardassians. They have spoons on their foreheads.
D is for Deneb IV. James Kirk and Gary Mitchell visited it more than a century before Jean-Luc Picard did.
E is for Ekos and it's Nazi copycats.
F is for the Federation Science Council.
G is for Gowron Chancellor of the Klingon Empire whom Worf killed in hand to hand combat in the DS9 season 7 episode Tacking Into The Wind
H is for Hengist, the human chief administrator of Argelius II in 2267 and the last known host of the Redjac entity.
I is Idanian Spice pudding which Quark brought to a party in the Ward Room in the DS9 season 3 episode Fascination
J is for Joachim (or is it "Joaquim"?) who was with Khan; depending on the name, the guy in the TOS episode, or the guy in the big screen sequel.
K is for Klingon tea drinking ritual.
L is for Larry Marvick. More proof that Starfleet needs to psych test it's employees better.
M is for Mellitus, a gaseous entity that became temporarily solid when in a resting state.
N is for Nerds on the trekbbs.
O is for Orbs, powerful spatial and temporal devices from the Wormhole aliens that were revered by Bajoran culture.
P is for Platonians. They discriminated against midgets. Heightists?