Although the general public think of the hairs, the anatomists and surgeons think of the region that bears them.
The skull is ridged above the orbit, the frontal eminence, more markedly in males than females and one of the features distinguishing the sexes.
The skin overlying the ridge is also thickened, in comparison with elsewhere in the forehead, and it is this arched pad of thickened tissues that the anatomist calls the eyebrow.
There is an arch of hairs of lengths varying with sex and age that are inserted into these pads, but whether the hairs are present or not (they are lost in some disease states), the region over the orbits is still the “eyebrow.”
Into the skin of the eyebrow pad are inserted muscle fibers from the frontal belly of occipito-frontalis and from the corrugator muscle. The former prevents the pad from drooping over the orbit, corrugator will “knit the brows” drawing the pads together and wrinkling the skin between them, over the root of the nose.
There is a great deal of teleological, hence unscientific, discussion about why we have the so-called eyebrows, and what purpose they might serve. What we do agree about is that women will commonly reshape them, and that might make them more attractive, whereas men are expected to tolerate them as they are. The hairs change with age, losing their well-organized state of youth to becoming longer and bushier. With age (also with associated obesity) the pad of tissue, the real eyebrow, thickens further and begins to descend over the upper eyelid.