The Collaborative International Dictionary of English v.0.48:

Sole \Sole\, n. [F. sole, L. solea; -- so named from its flat
   shape. See Sole of the foot.] (Zool.)
   (a) Any one of several species of flatfishes of the genus
       Solea and allied genera of the family Soleidae,
       especially the common European species (Solea
       vulgaris), which is a valuable food fish.
   (b) Any one of several American flounders somewhat resembling
       the true sole in form or quality, as the California sole
       (Lepidopsetta bilineata), the long-finned sole
       (Glyptocephalus zachirus), and other species.
       [1913 Webster]

   Lemon, or French, sole (Zool.), a European species of
      sole (Solea pegusa).

   Smooth sole (Zool.), the megrim.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lemon \Lem"on\ (l[e^]m"[u^]n), n. [F. limon, Per. l[imac]m[=u]n;
   cf. Ar. laim[=u]n, Sp. limon, It. limone. Cf. Lime a
   fruit.]
   [1913 Webster]
   1. (Bot.) An oval or roundish fruit resembling the orange,
      and containing a pulp usually intensely acid. It is
      produced by a tropical tree of the genus Citrus, the
      common fruit known in commerce being that of the species
      Citrus Limonum or Citrus Medica (var. Limonum). There
      are many varieties of the fruit, some of which are sweet.
      [1913 Webster]

   2. The tree which bears lemons; the lemon tree.
      [1913 Webster]

   Lemon grass (Bot.), a fragrant East Indian grass
      (Andropogon Sh[oe]nanthus, and perhaps other allied
      species), which yields the grass oil used in perfumery.

   Lemon sole (Zool.), a yellow European sole (Solea
      aurantiaca).

   Salts of lemon (Chem.), a white crystalline substance,
      inappropriately named, as it consists of an acid potassium
      oxalate and contains no citric acid, which is the
      characteristic acid of lemon; -- called also salts of
      sorrel. It is used in removing ink stains. See Oxalic
      acid, under Oxalic. [Colloq.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

lemon
    n 1: yellow oval fruit with juicy acidic flesh
    2: a strong yellow color [syn: gamboge, lemon, lemon
       yellow, maize]
    3: a small evergreen tree that originated in Asia but is widely
       cultivated for its fruit [syn: lemon, lemon tree, Citrus
       limon]
    4: a distinctive tart flavor characteristic of lemons
    5: an artifact (especially an automobile) that is defective or
       unsatisfactory [syn: lemon, stinker]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "lemon":
   acid, acidulant, aureate, auric, beige, bomb,
   bread-and-butter pickle, buff, buff-yellow, bust, canary,
   canary-yellow, chokecherry, citron, citron-yellow, crab apple,
   cream, creamy, dill pickle, dud, dull thud, ecru, fallow, fizzle,
   flat failure, flaxen, flop, floperoo, frost, gilded, gilt, gold,
   gold-colored, golden, green apple, lemon-yellow, lime, loser,
   luteolous, lutescent, ocherish, ocherous, ochery, ochreous,
   ochroid, ochrous, ochry, or, pickle, primrose, primrose-colored,
   primrose-yellow, saffron, saffron-colored, saffron-yellow, sallow,
   sand-colored, sandy, sour, sour balls, sour cream, sour grapes,
   sour pickle, sourdough, straw, straw-colored, total loss, turkey,
   verjuice, vinegar, washout, xanthic, xanthous, yellow, yellowish,
   yogurt