The Collaborative International Dictionary of English v.0.48:

Lentil \Len"til\ (l[e^]n"t[i^]l), n. [F. lentille, fr. L.
   lenticula, dim. of lens, lentis, lentil. Cf. Lens.] (Bot.)
   A leguminous plant of the genus Ervum (Ervum Lens), of
   small size, common in the fields in Europe. Also, its seed,
   which is used for food on the continent.
   [1913 Webster]

   Note: The lentil of the Scriptures probably included several
         other vetchlike plants.
         [1913 Webster]

   Lentil shell (Zool.), a small bivalve shell of the genus
      Ervillia, family Tellinid[ae].
      [1913 Webster]

WordNet (r) 3.0 (2006):

lentil
    n 1: round flat seed of the lentil plant used for food
    2: the fruit or seed of a lentil plant
    3: widely cultivated Eurasian annual herb grown for its edible
       flattened seeds that are cooked like peas and also ground
       into meal and for its leafy stalks that are used as fodder
       [syn: lentil, lentil plant, Lens culinaris]