The line of hair above each eye at the bottom of the forehead. They come in all shapes, sizes & styles. It serves the purpose of blocking sunlight from the eyes (notice how the eyes are farther back in the skull). People with darker colored eyebrows usually have better vision on a bright day, seeing as how dark colors (especially black) hold light, while blond might spread light. In my humble opinion, one of the sexiest places to get a piercing. When the two are connected by more hair, it is called a unibrow.
I can see better on a bright day than a blond because my eyebrows are black. And the chicks think my piercing is hot.