Definition of "first base"

first base

noun
countable and uncountable, plural first bases
1. (singular only, baseball) The base after home plate in a counter-clockwise path around a baseball infield.
2. (singular only, by extension) Completion of the first phase of an activity.
3. (singular only, US, colloquial) Kissing, regarded as the first phase of a sexual relationship.
  Quotations
  Every time I got to the part about her out with Stradlater in that damn Ed Banky's car, it almost drove me crazy. I knew she wouldn’t let him get to first base with her, but it drove me crazy anyway.
  1951, J. D. Salinger, The Catcher in the Rye, Little, Brown and Company, page 104
4. (blackjack) The betting spot located immediately to the left of the dealer, which is first to receive cards and first to act.