The Collaborative International Dictionary of English v.0.48:

Jam \Jam\, v. t. [imp. & p. p. Jammed (j[a^]md); p. pr. & vb.
   n. Jamming.] [Either fr. jamb, as if squeezed between
   jambs, or more likely from the same source as champ See
   Champ.]
   [1913 Webster]
   1. To press into a close or tight position; to crowd; to
      squeeze; to wedge in; to cram; as, rock fans jammed the
      theater for the concert.
      [1913 Webster]

            The ship . . . jammed in between two rocks. --De
                                                  Foe.
      [1913 Webster]

   2. To crush or bruise; as, to jam a finger in the crack of a
      door. [Colloq.]
      [1913 Webster]

   3. (Naut.) To bring (a vessel) so close to the wind that half
      her upper sails are laid aback. --W. C. Russell.
      [1913 Webster]

   4. To block or obstruct by packing too much (people or
      objects) into; as, shoppers jammed the aisles during the
      fire sale.
      [PJC]

   5. (Radio) To interfere with (a radio signal) by sending
      other signals of the same or nearby frequency; as, the
      Soviets jammed Radio Free Europe broadcasts for years
      during the cold war.
      [PJC]

   6. To cause to become nonfunctional by putting something in
      that blocks the movement of a part or parts; as, he jammed
      the drawer by putting in too many loose papers; he jammed
      the lock by trying to pick it.
      [PJC]

WordNet (r) 3.0 (2006):

jamming
    n 1: deliberate radiation or reflection of electromagnetic
         energy for the purpose of disrupting enemy use of
         electronic devices or systems [syn: jamming, electronic
         jamming, jam]