A is for Alexander the Platonian midget.
B is for the Beta Quadrant. The Klingon and Romulan Empires were located in both the Beta and Alpha Quadrants, straddling the commonly-accepted border between the two.
C is for Celestial Temple. AKA the Bajoran Wormhole.
D is for Doomsday Machine
E is for the Earth Starfleet, which changed to become the new unified, interspecies Federation Starfleet in 2161.
F is for freedom, a worship word of the Yangs.
G is for Garak. Plain? Simple? Not really. But humor him.
H is for Holodecks. And the silly things that go on in them. Someone needs to hardwire an off-switch on the bridge for those things.
I is for Independence. The Kriosians won theirs from the Klingon Empire in 2367.
J is for J, the letter of the USS Enterprise (NCC-1701-J). A ship from a possible future, that we didn't get to see enough of.
K is for Klingon occupation troops on Organia in the year 2267.
L is for Levinius V.
M is for Martian orbit, where the Ares IV manned mission to the planet in the year 2032 mysteriously vanished without a trace.
N is for Nagilum.
O is for Oberth-class science ships of the 23rd and 24th centuries.
P is for Pah-Wraiths. AKA wormhole aliens