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 grass
    n 1: an aromatic oil that smells like lemon and is widely used
         in Asian cooking and in perfumes and medicines [syn:
         lemongrass, lemon grass, lemongrass oil]
    2: a tropical grass native to India and Sri Lanka [syn:
       lemongrass, lemon grass]