first base [countable]
the first of the four places in a game of baseball that a player must touch before gaining a point
the position of a defending player near this place:
He plays first base for the Red Sox.
2 American English informal
to reach the first stage of success in an attempt to achieve something:
You've gotten to first base if you've landed an interview.
b) old-fashioned informal
to kiss someone in a sexual way, used especially by young men to talk about the first stage in a sexual relationship