The Collaborative International Dictionary of English v.0.48:

Rubber \Rub"ber\, n.
   1. One who, or that which, rubs. Specifically:
      (a) An instrument or thing used in rubbing, polishing, or
          cleaning.
      (b) A coarse file, or the rough part of a file.
      (c) A whetstone; a rubstone.
      (d) An eraser, usually made of caoutchouc or a synthetic
          rubber[4].
      (e) The cushion of an electrical machine.
      (f) One who performs massage, especially in a Turkish
          bath.
      (g) Something that chafes or annoys; hence, something that
          grates on the feelings; a sarcasm; a rub. --Thackeray.
          [1913 Webster]

   2. In some games, as bridge or whist, the odd game, as the
      third or the fifth, which decides the winner when there is
      a tie between the players; as, to play the rubber; also, a
      contest determined by the winning of two out of three
      games; as, to play a rubber of whist. --Beaconsfield. "A
      rubber of cribbage." --Dickens.
      [1913 Webster]

   3. India rubber; caoutchouc; gum elastic; -- also called
      natural rubber.
      [1913 Webster +PJC]

   4. Any substance, whether natural or synthetic, resembling
      India rubber with respect to its elasticity[1].
      [PJC]

   5. A low-cut overshoe made of natural or synthetic rubber[4],
      serving to keep the feet and shoes dry when walking in the
      rain or on a wet surface; -- usually used in the plural.
      [1913 Webster]

   6. A condom. [Slang]
      [PJC]

   Antimony rubber, an elastic durable variety of vulcanized
      caoutchouc of a red color. It contains antimony sulphide
      as an important constituent.

   Hard rubber, a kind of vulcanized caoutchouc which nearly
      resembles horn in texture, rigidity, etc.

   India rubber, caoutchouc. See Caoutchouc.

   Rubber cloth, cloth covered with caoutchouc for excluding
      water or moisture.

   Rubber dam (Dentistry), a shield of thin sheet rubber
      clasped around a tooth to exclude saliva from the tooth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

natural rubber
    n 1: an elastic material obtained from the latex sap of trees
         (especially trees of the genera Hevea and Ficus) that can
         be vulcanized and finished into a variety of products [syn:
         rubber, natural rubber, India rubber, gum elastic,
         caoutchouc]