Any creature not played as a character. NPC's are technically monsters. Killing monsters is the primary way to gain experience.

an imaginary creature usually having various human and animal parts

a creature with no consciousness

a legendary creature that inspires fear or horror

an alternative being, rather than an alternative state of consciousness

an NPC that exists for the sole purpose of being killed by PCs

a pretty sly creature

Any creature or character not controlled by a player. (D&D 1)

an NPC that most likely will attack you

Monster is a term for any number of legendary creatures that frequently appear in mythology, legend, and horror fiction. The word originates from the ancient Latin monstros, monstrum, from the root of monere, "to warn", also meaning prodigy, miracle.