What is a pit, groove, cavity, or depression, of greater or less depth; as, the temporal fossa on the side of the skull; the nasal fossæ containing the nostrils in most birds called?

fossa — A pit, groove, cavity, or depression, of greater or less depth; as, the temporal fossa on the side of the skull; the nasal fossæ containing the nostrils in most birds. (Webster's 1913.)