Source : Webster's Revised Unabridged Dictionary (1913)

Bunion \Bun"ion\, n. (Med.)
   Same as {Bunyon}.

Bunyon \Bun"yon\, Bunion \Bun"ion\, n. [Cf. Prov. E. bunny a
   small swelling, fr. OF. bugne, It. bugna, bugnone. See
   {Bun}.] (Med.)
   An enlargement and inflammation of a small membranous sac
   (one of the burs[ae] muscos[ae]), usually occurring on the
   first joint of the great toe.

Source : WordNet®

bunion
     n : a painful swelling of the bursa of the first joint of the
         big toe