What is first base second base and third base in a relationship?

1st base is kissing. 2nd base is kissing and petting each other. Getting to3rd base refers to oral sex. The idiomatic expressions are based on the great national pastime of Americans, baseball. In one Seinfeld episode, Elaine puns on the expression by telling baseball legend Keith Hernandez that he may be a great baseball player, but he'll never get to 3rd base with her.