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]

WordNet (r) 3.0 (2006):

fossa
    n 1: a concavity in a surface (especially an anatomical
         depression) [syn: pit, fossa]
    2: monotypic genus of Madagascar civets closely related to palm
       civets [syn: Fossa, genus Fossa]
    3: largest carnivore of Madagascar; intermediate in some
       respects between cats and civets [syn: fossa, fossa cat,
       Cryptoprocta ferox]