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.