Among American adolescents, baseball metaphors for sex are often used as euphemisms for the degree of physical intimacy achieved in sexual encounters or relationships.In the metaphor, first prevalent in the aftermath of World War II, sexual activities are described as if they are actions in a game of baseball.

Among the most commonly used metaphors are the bases describing levels of physical intimacy (generally from a heterosexual perspective). Definitions vary, but the following are typical usages of the terms:

1. First base – mouth-to-mouth kissing, especially French kissing;

2. Second base – skin-to-skin touching/kissing of the breasts; in some contexts, it may instead refer to touching any erogenous zones
through the clothes (i.e. not actually touching the skin), or manual stimulation of the genitals;

3. Third base – touching below the waist (without sexual intercourse); in some contexts, it may instead refer to oral stimulation of the genitals;
Home run (home base) – "full" sexual intercourse.

Additionally:

4. Strikeout – a failure to engage in any form of pre-intimacy or other sexual activity;

Additionally, there are two metaphors used to refer to men who have sex with men:

5. Pitching – the man performing anal sex

6. Catching – the man receiving anal sex

Other similar metaphors include:

Switch hitter – a bisexual individual, referencing a player who can bat from either side
Playing for the other team – usually indicating a person is gay or lesbian
Playing for both teams – usually indicating a person is bisexual

The metaphors are found variously in popular American culture, with one well-known example in the Meat Loaf song "Paradise by the Dashboard Light", which describes a young couple "making out", with a voice-over commentary, by baseball announcer Phil Rizzuto, of a portion of a baseball game as a metaphor for the couple's activities. A similar example can be found in Billy Joel's song "Zanzibar" in which he compares himself to Pete Rose and sings the lines "Me, I'm trying just to get to second base and I'd steal it if she only gave the sign. She's gonna give the go ahead, the inning isn't over yet for me."