third base (Noun) — The base that must be touched third by a base runner in baseball. ex. "he was cut down on a close play at third base"
third base (Noun) — The fielding position of the player on a baseball team who is stationed near the third of the bases in the infield (counting counterclockwise from home plate). ex. "he is playing third base"
third base (Noun) — (informal) sexual touching of genitals; mutual masturbation.