The glabella is the part of your forehead that's between your eyebrows. It might refer to the smooth bone beneath the skin, or it might refer to the skin and soft tissue covering the bone.
The word glabella comes from the Latin adjective glabellus, which means "hairless" or "smooth." Ironically, if you have a unibrow, your glabella might be hidden — by hair! If you've been at a loss to describe the phenomenon of the unibrow (also called the monobrow), you can now define it as "eyebrows joined by a growth of hair across the glabella."