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

The Collaborative International Dictionary of English v.0.48:

Pearl \Pearl\, n. [OE. perle, F. perle, LL. perla, perula,
   probably fr. (assumed) L. pirulo, dim. of L. pirum a pear.
   See Pear, and cf. Purl to mantle.]
   1. (Zool.) A shelly concretion, usually rounded, and having a
      brilliant luster, with varying tints, found in the mantle,
      or between the mantle and shell, of certain bivalve
      mollusks, especially in the pearl oysters and river
      mussels, and sometimes in certain univalves. It is usually
      due to a secretion of shelly substance around some
      irritating foreign particle. Its substance is the same as
      nacre, or mother-of-pearl.

   Note: Pearls which are round, or nearly round, and of fine
         luster, are highly esteemed as jewels, and at one time
         compared in value with the precious stones. Since
         development of cultured pearls, the relative value has
         diminished somewhat, though the best pearls are still
         expensive, and natural pearls even more so. Artificial
         pearls may be made of various materials, including
         material similar to that of natural pearls; these are
         less expensive than natural or cultured pearls. See
         cultured pearl, below.
         [1913 Webster +PJC]

   2. Hence, figuratively, something resembling a pearl;
      something very precious.
      [1913 Webster]

            I see thee compassed with thy kingdom's pearl.
                                                  --Shak.
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            And those pearls of dew she wears.    --Milton.
      [1913 Webster]

   3. Nacre, or mother-of-pearl.
      [1913 Webster]

   4. (Zool.) A fish allied to the turbot; the brill.
      [1913 Webster]

   5. (Zool.) A light-colored tern.
      [1913 Webster]

   6. (Zool.) One of the circle of tubercles which form the bur
      on a deer's antler.
      [1913 Webster]

   7. A whitish speck or film on the eye. [Obs.] --Milton.
      [1913 Webster]

   8. A capsule of gelatin or similar substance containing some
      liquid for medicinal application, as ether.
      [1913 Webster]

   9. (Print.) A size of type, between agate and diamond.
      [1913 Webster]

   Ground pearl. (Zool.) See under Ground.

   Pearl barley, kernels of barley, ground so as to form
      small, round grains.

   Pearl diver, one who dives for pearl oysters.

   Pearl edge, an edge of small loops on the side of some
      kinds of ribbon; also, a narrow kind of thread edging to
      be sewed on lace.

   Pearl eye, cataract. [R.]

   Pearl gray, a very pale and delicate blue-gray color.

   Pearl millet, Egyptian millet (Penicillaria spicata).

   Pearl moss. See Carrageen.

   Pearl moth (Zool.), any moth of the genus Margaritia; --
      so called on account of its pearly color.

   Pearl oyster (Zool.), any one of several species of large
      tropical marine bivalve mollusks of the genus
      Meleagrina, or Margaritifera, found in the East Indies
      (especially at Ceylon), in the Persian Gulf, on the coast
      of Australia, and on the Pacific coast of America. Called
      also pearl shell, and pearl mussel.

   Pearl powder. See Pearl white, below.

   Pearl sago, sago in the form of small pearly grains.

   Pearl sinter (Min.), fiorite.

   Pearl spar (Min.), a crystallized variety of dolomite,
      having a pearly luster.

   Pearl white.
      (a) Basic bismuth nitrate, or bismuth subchloride; -- used
          chiefly as a cosmetic.
      (b) A variety of white lead blued with indigo or Berlin
          blue.

   cultured pearl, a pearl grown by a pearl oyster into which
      a round pellet has been placed, to serve as the seed for
      more predictable growth of the pearl. The pellet is
      usually made from mother-of-pearl, and additional layers
      of nacre are deposited onto the seed by the oyster. Such
      pearls, being more easily obtained than natural pearls
      from wild oysters, are less expensive.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pearl oyster
    n 1: tropical marine bivalve found chiefly off eastern Asia and
         Pacific coast of North America and Central America; a major
         source of pearls [syn: pearl oyster, Pinctada
         margaritifera]