First base: Kissing (with tongue).

Second base: Rubbing or fondling a girl's breasts. (Some hardcore fanatics used to insist that getting to second base required going under the shirt and unhooking the bra. Less staunch definitions allowed feeling the boobies over the shirt).

Third base: Stimulating the vulva. (Unrestricted access required in all schools of thought. Any material posing as a barrier, whether denim, cotton or silk, disqualified you from being able to say 'I got to third base!'. If you reached third you had probably put a lot of time into the relationship, and thus tricked the girl into thinking you were a nice guy. A smelly finger was required as proof).

Homerun: Actual vaginal penetration. Minimum of one pump required (by both schools) before prematurely ejaculating and saying, "Wow, I never got off that fast before. You're something special."

Footnotes:

Dry humping never earned its own base because it was practiced by many couples who never even reached second base.

A hand job was like being caught in a rundown between third and home plate. After a hand job many boys were unable to masturbate for days due to the torture inflicted on their poor little willies by girls who thought they were speed-shifting in the friggin' Indy 500.

Oral sex was considered a Grand slam. Unfortunately, it only came at the stage of the relationship where, A: the girl started talking about marriage. Or B: your relationship was in the pits and she just started dating someone secretly and was using you for; A: Practice. Or B: To show you what you would soon be missing. The only time oral sex was not considered a Grand slam was when you got it from a girl at a party who had blown five other guys beside yourself.