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]

The Collaborative International Dictionary of English v.0.48:

Lime \Lime\, n. [F. lime; of Persian origin. See Lemon.]
   1. (Bot.) The fruit of the Citrus aurantifolia, allied to
      the lemon, but greener in color; also, the tree which
      bears it.

   Note: The term lime was formerly also applied to variants of
         the closely related citron, of which there are two
         varieties, Citrus Medica, var. acida which is
         intensely sour, and the

   sweet lime (Citrus Medica, var. Limetta) which is only
      slightly sour. See citron.
      [1913 Webster +PJC]

   2. The color of the lime[1], a yellowish-green.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Cedrat \Ce"drat\ (s[=e]"dr[a^]t), n. [Cf. F. c['e]drat. See
   Cedar.] (Bot.)
   Properly the citron, a variety of Citrus medica, with large
   fruits, not acid, and having a high perfume.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Citron \Cit"ron\ (s[i^]t"r[u^]n), n. [F. citron, LL. citro, fr.
   L. citrus citron tree (cf. citreum, sc. malum, a citron),
   from Gr. ki`tron citron]
   1. (Bot) A fruit resembling a lemon, but larger, and
      pleasantly aromatic; it is produced by the citron tree
      (Citrus medica). The thick rind, when candied, is the
      citron of commerce. The fruit was once called the lime.
      [1913 Webster]

   2. A citron tree, Citrus medica.
      [1913 Webster]

   3. A citron melon.
      [1913 Webster]

   Citron melon.
      (a) A small variety of muskmelon with sugary greenish
          flesh.
      (b) A small variety of watermelon, whose solid white flesh
          is used in making sweetmeats and preserves.

   Citron tree (Bot.), the tree which bears citrons. It was
      probably a native of northern India, and is now understood
      to be the typical form of Citrus Medica.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Citrus medica
    n 1: thorny evergreen small tree or shrub of India widely
         cultivated for its large lemonlike fruits that have thick
         warty rind [syn: citron, citron tree, Citrus medica]