Enemy

An enemy, for the purpose of combat targeting, is:
 * 1) Any mob (or their pet) with which you have less than friendly faction reputation (neutral, unfriendly, hostile and hated).
 * 2) A PvP flagged player (or their pet) of the opposite faction.
 * 3) Any player (or their pet) you are currently dueling.
 * 4) Critters. Those poor squirrels.

Note that it is possible, through reputation changes, for friendly NPCs to become ememies and vice versa.

More generally, "enemy" may refer to the faction opposite the player's own (e.g. Horde for a night elf), or any faction opposed to the player's, regardless of attackable status.