The Collaborative International Dictionary of English v.0.48:

Jump \Jump\, n.
   1. The act of jumping; a leap; a spring; a bound. "To advance
      by jumps." --Locke.
      [1913 Webster]

   2. An effort; an attempt; a venture. [Obs.]
      [1913 Webster]

            Our fortune lies
            Upon thisjump.                        --Shak.
      [1913 Webster]

   3. The space traversed by a leap.
      [1913 Webster]

   4. (Mining) A dislocation in a stratum; a fault.
      [1913 Webster]

   5. (Arch.) An abrupt interruption of level in a piece of
      brickwork or masonry.
      [1913 Webster]

   6. A jump-start; as, to get a jump from a passing mmotorist.
      [PJC]

   From the jump, from the start or beginning. [Colloq.]

   Jump joint.
      (a) A butt joint.
      (b) A flush joint, as of plank in carvel-built vessels.

   Jump seat.
      (a) A movable carriage seat.
      (b) A carriage constructed with a seat which may be
          shifted so as to make room for second or extra seat.
          Also used adjectively; as, a jump-seat wagon.
          [1913 Webster]

WordNet (r) 3.0 (2006):

jump seat
    n 1: a folding seat in an automobile