Source : Webster's Revised Unabridged Dictionary (1913)

Antrum \An"trum\, n.; pl. {Antra}. [L., fr. Gr. ?.]
   A cavern or cavity, esp. an anatomical cavity or sinus.
   --Huxley.

Source : WordNet®

antra
     See {antrum}

antrum
     n : a natural cavity or hollow in a bone
     [also: {antra} (pl)]