The Collaborative International Dictionary of English v.0.48:

Tractor \Tract"or\, n. [NL., from L. trahere, tractum, to draw.]
   1. That which draws, or is used for drawing.
      [1913 Webster]

   2. pl. (Med.) Two small, pointed rods of metal, formerly used
      in the treatment called Perkinism.
      [1913 Webster]

   3. a motor vehicle with a powerful engine and large wheels,
      used to pull farm equipment such as plows, harvesting
      machinery, etc., over farm land. Smaller forms of farm
      tractor are completely open, with only a seat for the
      driver.
      [PJC]

   4. a motor vehicle with a driver's cab and a powerful engine,
      fitted with a coupling at the rear, and designed to pull a
      large trailer or semi-trailer on the open highway; --
      called also truck tractor. The combination of such a
      tractor with its trailer is called a tractor-trailer, and
      is a common form of truck used for commercial shipping of
      freight.
      [PJC]

   5. (Aviation) A propeller screw placed in front of the
      supporting planes of an aeroplane instead of behind them,
      so that it exerts a pull instead of a push; -- called also
      Tractor screw and Tractor propeller.
      [Webster 1913 Suppl.]

   6. (Aviation) An airplane having one or more tractor
      propellers; -- called also tractor airplane.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Tractor screw \Tractor screw\ or Tractor propeller \Tractor
propeller\ (Aviation)
   A propeller screw placed in front of the supporting planes of
   an airplane instead of behind them, so that it exerts a pull
   instead of a push. Hence,

   Tractor monoplane,

   Tractor biplane, etc. This is the most common arrangement
      for propellers on modern airplanes.
      [Webster 1913 Suppl.]