The Collaborative International Dictionary of English v.0.48:

Yellowtail \Yel"low*tail`\, n. (Zool.)
   (a) Any one of several species of marine carangoid fishes of
       the genus Seriola; especially, the large California
       species (Seriola dorsalis) which sometimes weighs
       thirty or forty pounds, and is highly esteemed as a food
       fish; -- called also cavasina, and white salmon.
   (b) The mademoiselle, or silver perch.
   (c) The menhaden.
   (d) The runner, 12.
   (e) A California rockfish (Sebastodes flavidus).
   (f) The sailor's choice (Diplodus rhomboides).
       [1913 Webster]

   Note: Several other fishes are also locally called
         yellowtail.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Seriola dorsalis
    n 1: game fish of southern California and Mexico having a yellow
         tail fin [syn: yellowtail, Seriola dorsalis]