18 minus epsilon

Refers to a girl who isn't quite legal, but, come on bro, she basically is. She is weeks away from being legal, and can be easily mistaken for 22 by your average xanax-ed out celebrity. Comes from mathematical analysis, where the phrase "minus epsilon" means "minus some arbitrarily small quantity".

Dude 1: "Did you hear that James Franco was trying to pick up a seventeen year old?" Dude 2: "But did you see her? She was totally 18 minus epsilon".