The Collaborative International Dictionary of English v.0.48:

Branchia \Bran"chi*a\, n.; pl. Branchi[ae]. [L., fr. Gr. ?,
   pl. of ?.] (Anat.)
   A gill; a respiratory organ for breathing the air contained
   in water, such as many aquatic and semiaquatic animals have.
   [1913 Webster]

WordNet (r) 3.0 (2006):

branchia
    n 1: respiratory organ of aquatic animals that breathe oxygen
         dissolved in water [syn: gill, branchia]