The Collaborative International Dictionary of English v.0.48:

Oyster \Oys"ter\ (ois"t[~e]r), n. [OF. oistre, F. hu[^i]tre, L.
   ostrea, ostreum, Gr. 'o`streon; prob. akin to 'ostre`on bone,
   the oyster being so named from its shell. Cf. Osseous,
   Ostracize.]
   1. (Zool.) Any marine bivalve mollusk of the genus Ostrea.
      They are usually found adhering to rocks or other fixed
      objects in shallow water along the seacoasts, or in
      brackish water in the mouth of rivers. The common European
      oyster (Ostrea edulis), and the American oyster (Ostrea
      Virginiana), are the most important species.
      [1913 Webster]

   2. A name popularly given to the delicate morsel contained in
      a small cavity of the bone on each side of the lower part
      of the back of a fowl.
      [1913 Webster]

   Fresh-water oyster (Zool.), any species of the genus
      Etheria, and allied genera, found in rivers of Africa
      and South America. They are irregular in form, and attach
      themselves to rocks like oysters, but they have a pearly
      interior, and are allied to the fresh-water mussels.

   Oyster bed, a breeding place for oysters; a place in a
      tidal river or other water on or near the seashore, where
      oysters are deposited to grow and fatten for market. See
      1st Scalp, n.

   Oyster catcher (Zool.), See oystercatcher in the
      vocabulary.

   Oyster crab (Zool.) a small crab (Pinnotheres ostreum)
      which lives as a commensal in the gill cavity of the
      oyster.

   Oyster dredge, a rake or small dragnet for bringing up
      oysters from the bottom of the sea.

   Oyster fish. (Zool.)
      (a) The tautog.
      (b) The toadfish.

   Oyster plant. (Bot.)
      (a) A plant of the genus Tragopogon (Tragopogon
          porrifolius), the root of which, when cooked,
          somewhat resembles the oyster in taste; salsify; --
          called also vegetable oyster.
      (b) A plant found on the seacoast of Northern Europe,
          America and Asia (Mertensia maritima), the fresh
          leaves of which have a strong flavor of oysters.

   Oyster plover. (Zool.) Same as oystercatcher.

   Oyster shell (Zool.), the shell of an oyster.

   Oyster wench, Oyster wife, Oyster women, a women who
      deals in oysters.

   Pearl oyster. (Zool.) See under Pearl.

   Thorny oyster (Zool.), any spiny marine shell of the genus
      Spondylus.
      [1913 Webster] oystercatcher

WordNet (r) 3.0 (2006):

Pinnotheres ostreum
    n 1: tiny soft-bodied crab living within the mantle cavity of
         oysters [syn: oyster crab, Pinnotheres ostreum]