The Collaborative International Dictionary of English v.0.48:

Patella \Pa*tel"la\, n.; pl. Patell[ae]. [L., a small pan, the
   kneepan, dim. of patina, patena, a pan, dish.]
   1. A small dish, pan, or vase.
      [1913 Webster]

   2. (Anat.) The kneecap; the kneepan; the cap of the knee.
      [1913 Webster]

   3. (Zool.) A genus of marine gastropods, including many
      species of limpets. The shell has the form of a flattened
      cone. The common European limpet (Patella vulgata) is
      largely used for food.
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   4. (Bot.) A kind of apothecium in lichens, which is
      orbicular, flat, and sessile, and has a special rim not a
      part of the thallus.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Limpet \Lim"pet\ (l[i^]m"p[e^]t), n. [Prob. through French fr.
   L. lepas, -adis, Gr. lepa`s, -a`dos.] (Zool.)
   1. In a general sense, any hatshaped, or conical, gastropod
      shell.
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   2. Any one of many species of marine shellfish of the order
      Docoglossa, mostly found adhering to rocks, between
      tides.
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   Note: The common European limpets of the genus Patella
         (esp. Patella vulgata) are extensively used as food.
         The common New England species is Acm[ae]a
         testudinalis. Numerous species of limpets occur on the
         Pacific coast of America, some of them of large size.
         [1913 Webster]

   3. Hence: Somthing or someone that clings tenaciously to
      another object or person; specifically a military
      explosive device having magnets allowing it to cling to a
      metallic target object, such as the hull of a ship.
      [PJC]

   3. Any species of Siphonaria, a genus of limpet-shaped
      Pulmonifera, living between tides, on rocks.
      [1913 Webster]

   4. A keyhole limpet. See Fissurella.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Patella vulgata
    n 1: marine limpet [syn: common limpet, Patella vulgata]