Overhead has one meaning and one meaning only in the surfing world. It means: a wave that is taller than you are. Practically since the invention of the sport, surfers have argued over the best way to measure the height of a wave. Some say it's measured from the front, while some say it's measured from the back. So while one guy says, "Hey, the waves are like 6 feet today" his buddy might say, "Um, no, it's barely 3 feet."
Source: Yahoo: Answers, Facebook: JimmyLewisBoards, TheSurfingSite: Wave Height