The Collaborative International Dictionary of English v.0.48:

Cucumber \Cu"cum*ber\ (k?`k?m-b?r, formerly kou"k?m-b?r), n.
   [OE. cucumer, cocumber, cucumber, fr. L. cucmis,
   gen.cucumeris; cf. OF. cocombre,F. concombre.] (Bot.)
   A creeping plant, and its fruit, of several species of the
   genus Cucumis, esp. Cucumis sativus, the unripe fruit of
   which is eaten either fresh or picked. Also, similar plants
   or fruits of several other genera. See below.
   [1913 Webster]

   Bitter cucumber (Bot.), the Citrullus Colocynthis syn.
      Cucumis Colocynthis. See Colocynth.

   Cucumber beetle. (Zool.)
   (a) A small, black flea-beetle (Crepidodera cucumeris),
       which destroys the leaves of cucumber, squash, and melon
       vines.
   (b) The squash beetle.

   Cucumber tree.
   (a) A large ornamental or shade tree of the genus Magnolia
       (Magnolia acuminata), so called from a slight
       resemblance of its young fruit to a small cucumber.
   (b) An East Indian plant (Averrhoa Bilimbi) which produces
       the fruit known as bilimbi.

   Jamaica cucumber, Jerusalem cucumber, the prickly-fruited
      gherkin (Cucumis Anguria).

   Snake cucumber, a species (Cucumis flexuosus) remarkable
      for its long, curiously-shaped fruit.

   Squirting cucumber, a plant (Ecbalium Elaterium) whose
      small oval fruit separates from the footstalk when ripe
      and expels its seeds and juice with considerable force
      through the opening thus made. See Elaterium.

   Star cucumber, a climbing weed (Sicyos angulatus) with
      prickly fruit.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cucumber
    n 1: a melon vine of the genus Cucumis; cultivated from earliest
         times for its cylindrical green fruit [syn: cucumber,
         cucumber vine, Cucumis sativus]
    2: cylindrical green fruit with thin green rind and white flesh
       eaten as a vegetable; related to melons [syn: cucumber,
       cuke]