ˌfirst ˈbase noun [countable]

1.
a) the first of the four places in a game of baseball that a player must touch before gaining a point
b) the position of a defending player near this place:
He plays first base for the Red Sox.

2. get to/reach first base American English informal
a) 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

[TahlilGaran] Dictionary of Contemporary English ▲

first base
n. phr.
1. The base that must be touched first by a baseball player after batting.
He got to first base on four balls.
2. See: GET TO FIRST BASE.

[TahlilGaran] English Idioms Dictionary ▲