The Collaborative International Dictionary of English v.0.48:

defrost \defrost\ v. t.
   To remove the frost or ice from; as, to defrost the car
   window; to defrost a refrigerator.

   Syn: deice.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

defrost \defrost\ v. i.
   1. To become free of frost or ice; as, it took four hours for
      the refrigerator to defrost.

   Syn: deice.
        [PJC]

   2. To thaw; -- used especially of items removed from a
      freezer, such as frozen foods.
      [PJC]

WordNet (r) 3.0 (2006):

defrost
    v 1: make or become free of frost or ice; "Defrost the car
         window" [syn: defrost, deice, de-ice]