The Collaborative International Dictionary of English v.0.48:

Triple-tail \Tri"ple-tail`\, n. (Zool.)
   An edible fish (Lobotes Surinamensis) found in the warmer
   parts of all the oceans, and common on the southern and
   middle coasts of the United States. When living it is silvery
   gray, and becomes brown or blackish when dead. Its dorsal and
   anal fins are long, and extend back on each side of the tail.
   It has large silvery scales which are used in the manufacture
   of fancy work. Called also, locally, black perch,
   grouper, and flasher.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tripletail
    n 1: large food fish of warm waters worldwide having long anal
         and dorsal fins that with a caudal fin suggest a three-
         lobed tail