in Math, a prime number of the form, 2^(n) + 1, named after the famous 19th century mathematician Pierre de Fermat. However, in geek circles, it refers to the third nipple of women with a third nipple because 3 is the first Fermat Prime.
Geek 1: "I was going to sleep with Jessica from Real Analysis but it turns out she has a Fermat Prime."
Geek 2: "What dude? I was just trying to figure out the 12th Fermat Prime."