A is for Atomic bomb test. Quark and Odo used one to help propel their Ferengi shuttle from 1947 back to the late 24th century.
B is for Boothby. Starfleet Academy caretaker.
C is for c, the speed of light. The real-life maximum speed limit of the universe. Breaking it in Star Trek began the whole adventure.
D is for Deneb IV, visited by both James Kirk and Jean-Luc Picard during their respective Starfleet careers.