A 100 millionth of a Bitcoin, comminly known as the penny of bit coins, named after Bitcoins creator. It takes 2000-3000 of these to equal one $0.01. Also a common way to see the ammount of money your taking in while mining Bitcoin. Most computers ($1000ish) can get around 100-200 Satoshis an hour.
Guy 1: Dude i was mining Bitcoins last night
Guy 2: Same, I got a few Satoshi now