A plum was a type of plant fruit often eaten as food.
The word "plum" was used as an affectionate nickname given to Doctor Leonard McCoy by Nancy Crater when the two had a brief relationship in the 2250s. (TOS episode: "The Man Trap")
A type of tomato, the plum tomato, was named after this fruit because of a resemblance between the two (although no direct relation exists).
Types of plum