first base (Noun) — The base that must be touched first by a base runner in baseball.
first base (Noun) — The fielding position of the player on a baseball team who is stationed at first of the bases in the infield (counting counterclockwise from home plate).
first base (Noun) — The initial stage in accomplishing something. ex. "we didn't get to first base with that approach"
first base (Noun) — Affectionate play (or foreplay without contact with the genital organs).