The Collaborative International Dictionary of English v.0.48:

leatherjacket \leatherjacket\ n.
   1. any of several brightly colored tropical filefishes.

   Syn: leatherfish.
        [WordNet 1.5]

   2. any of several New World tropical fishes having tiny
      embedded scales.

   Syn: leatherjack.
        [WordNet 1.5]

   3. The tough-skinned larva of certain crane flies.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

leatherjacket
    n 1: any of several brightly colored tropical filefishes [syn:
         leatherjacket, leatherfish]
    2: any of several New World tropical fishes having tiny embedded
       scales [syn: leatherjacket, leatherjack]
    3: tough-skinned larva of certain crane flies