Also known as Trips, Thee of a Kind.
A set means a three of a kind. Generally, people refer to a set when someone has a three of a kind because they have a pocket pair, with a board matching that rank of the pocket pair in hold'em. For example, if I hold 77 and the flop comes J76, I flopped a set of sevens.
See also Quads.
(Submitted by TwoGun)