The Collaborative International Dictionary of English v.0.48:

Tussock \Tus"sock\, n. [From Tuz.] [Written also tussuck.]
   1. A tuft, as of grass, twigs, hair, or the like; especially,
      a dense tuft or bunch of grass or sedge.
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            Such laying of the hair in tussocks and tufts.
                                                  --Latimer.
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   2. (Bot.) Same as Tussock grass, below.
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   3. (Zool.) A caterpillar of any one of numerous species of
      bombycid moths. The body of these caterpillars is covered
      with hairs which form long tufts or brushes. Some species
      are very injurious to shade and fruit trees. Called also
      tussock caterpillar. See Orgyia.
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   Tussock grass. (Bot.)
      (a) A tall, strong grass of the genus Dactylis
          (Dactylis caespitosa), valuable for fodder,
          introduced into Scotland from the Falkland Islands.
      (b) A tufted grass (Aira caespitosa).
      (c) Any kind of sedge (Carex) which forms dense tufts in
          a wet meadow or boggy place.

   Tussock moth (Zool.), the imago of any tussock caterpillar.
      They belong to Orgyia, Halecidota, and allied genera.
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WordNet (r) 3.0 (2006):

tussock caterpillar
    n 1: larva of a tussock moth