The Collaborative International Dictionary of English v.0.48:

landing gear \land"ing gear`\, n.
   The wheels and attached structures under an airplane that
   support it and allow it to move when on the ground; also, the
   floats or pontoons of an amphibious airplane together with
   their supporting structures. Landing gear may be fixed
   rigidly in place, or retractable when in flight.
   [PJC]

WordNet (r) 3.0 (2006):

landing gear
    n 1: an undercarriage that supports the weight of the plane when
         it is on the ground