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.
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            I see thee compassed with thy kingdom's pearl.
                                                  --Shak.
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            And those pearls of dew she wears.    --Milton.
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   3. Nacre, or mother-of-pearl.
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   4. (Zool.) A fish allied to the turbot; the brill.
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   5. (Zool.) A light-colored tern.
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   6. (Zool.) One of the circle of tubercles which form the bur
      on a deer's antler.
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   7. A whitish speck or film on the eye. [Obs.] --Milton.
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   8. A capsule of gelatin or similar substance containing some
      liquid for medicinal application, as ether.
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   9. (Print.) A size of type, between agate and diamond.
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   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 millet
    n 1: tall grass having cattail like spikes; grown in Africa and
         Asia for its grain and in the United States chiefly for
         forage; sometimes used in making beer [syn: pearl millet,
         bulrush millet, cattail millet, Pennisetum glaucum,
         Pennisetum Americanum]