The Collaborative International Dictionary of English v.0.48:

Trailer \Trail"er\, n.
   1. a wheeled vehicle without a motor, designed to be drawn by
      a motor vehicle in front of it; specifically:
      (a) such a vehicle used on street railroads. Called also
          trail car.
      (b) the large wheeled wagon or van pulled by a tractor in
          a tractor-trailer combination.
      (c) a vehicle equipped as a mobile dwelling unit, pulled
          by an automobile or other mtor vehicle, and used as a
          dwelling when parked; -- also called a mobile home.
      (d) A wheeled motorless open wagon designed to carry a
          heavy object, such as a boat trailer.
          [PJC]

   2. (Movies) A short blank segment of movie film attached to
      the end; -- used for convenient insertion of the film in a
      projector.
      [PJC]

   3. (Movies) A short film consisting primarily of one or more
      short portions of a film, used in promotions or
      advertisements shortly before initial release of a film.
      [PJC]

   4. A part of an object which extends some distance beyond the
      main body of the object; as, the trailer of a plant.
      [1913 Webster]

   trailer park. An area equipped to accommodate trailers[2],
      often with outlets supplying electrical power and water.
      Called also trailer camp, trailer court.
      [PJC]