The Collaborative International Dictionary of English v.0.48:

fossa \fos"sa\, n.; pl. foss[ae]. [L., a ditch. See Fosse.]
   (Anat.)
   A pit, groove, cavity, or depression, of greater or less
   depth; as, the temporal fossa on the side of the skull; the
   nasal foss[ae] containing the nostrils in most birds.
   [1913 Webster]