I would just call it a bald spot. A typical, inherited type where the bald spot starts at the forehead and proceeds back across the top of the head is, I think, called "male-pattern baldness."
Actually, I wouldn't say that almost everyone has a bald spot -- maybe a spot where the whorls of hair are centered and the hair is a bit thinner, but I wouldn't call that a bald spot on most people. Maybe barbers or hairdressers have a special name for it.