A is for Atavachron. A temporal portal.
B is for the Beltane star system.
C is for Compressed teryon beam.
D is for the Duras sisters, Lursa and B'Etor. Just as dangerous as if not moreso than their late brother.
E is for Elasian females. Real fun on dates. Just don't let them pull their famous crying stunt on you.
F is for the Fabrini Oracle, the computerized - and authoritarian - ruler of the Fabrini people of Yonada during part of their journey through interstellar space.
G is for Guinan who works in Ten Forward.
H is for the Halkans. Treated with a measure of respect in our universe, threatened with complete and brutal annihilation in the Mirror Universe.
I is for Intendant Kira from the alternate timeline.
J is for James R. Kirk, the incorrect inscription on the Enterprise captain's tombstone that was created out of thin air by the mutated, power-mad Gary Mitchell during their climactic battle on the surface of Delta Vega.
K is for Klingons
L is for Lurians. Morn from Quark's Bar was one of them.
M is for Mutara Nebula.
N is for Nero, a Romulan miner of the Picard era of the Prime timeline who is accidentally thrown back in history to create a new timeline where he wreaks genocidal havoc.
O is for Oath of Celibacy. Pledge required of Starfleet personnel native to Delta IV.
P is for Porthos. Some alien species were put off that the Enterprise NX-01 had a quadraped crewmember that walked around naked and couldn't speak English.
Q is for Quadrotriticale.
R is for Reman dilithium mines, located deep underground and overseen by armed Romulan guards.
S is for Saucer Separation.
T is for the Tholian Assembly.
U is for Uhura's ASSets.