The Collaborative International Dictionary of English v.0.48:

Giant \Gi"ant\, a.
   Like a giant; extraordinary in size, strength, or power; as,
   giant brothers; a giant son.
   [1913 Webster]

   Giant cell. (Anat.) See Myeloplax.

   Giant clam (Zool.), a bivalve shell of the genus
      Tridacna, esp. T. gigas, which sometimes weighs 500
      pounds. The shells are sometimes used in churches to
      contain holy water.

   Giant heron (Zool.), a very large African heron (Ardeomega
      goliath). It is the largest heron known.

   Giant kettle, a pothole of very large dimensions, as found
      in Norway in connection with glaciers. See Pothole.

   Giant powder. See Nitroglycerin.

   Giant puffball (Bot.), a fungus (Lycoperdon giganteum),
      edible when young, and when dried used for stanching
      wounds.

   Giant salamander (Zool.), a very large aquatic salamander
      (Megalobatrachus maximus), found in Japan. It is the
      largest of living Amphibia, becoming a yard long.

   Giant squid (Zool.), one of several species of very large
      squids, belonging to Architeuthis and allied genera.
      Some are over forty feet long.
      [1913 Webster]

WordNet (r) 3.0 (2006):

giant clam
    n 1: a large clam inhabiting reefs in the southern Pacific and
         weighing up to 500 pounds [syn: giant clam, Tridacna
         gigas]