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]

The Collaborative International Dictionary of English v.0.48:

Giant \Gi"ant\, n. [OE. giant, geant, geaunt, OF. jaiant, geant,
   F. g['e]ant, L. gigas, fr. Gr. ?, ?, from the root of E.
   gender, genesis. See Gender, and cf. Gigantic.]
   [1913 Webster]
   1. A man of extraordinari bulk and stature.
      [1913 Webster]

            Giants of mighty bone and bold emprise. --Milton.
      [1913 Webster]

   2. A person of extraordinary strength or powers, bodily or
      intellectual.
      [1913 Webster]

   3. Any animal, plant, or thing, of extraordinary size or
      power.
      [1913 Webster]

   Giant's Causeway, a vast collection of basaltic pillars, in
      the county of Antrim on the northern coast of Ireland.
      [1913 Webster]

WordNet (r) 3.0 (2006):

giant
    adj 1: of great mass; huge and bulky; "a jumbo jet"; "jumbo
           shrimp" [syn: elephantine, gargantuan, giant,
           jumbo]
    n 1: any creature of exceptional size
    2: a person of exceptional importance and reputation [syn:
       colossus, behemoth, giant, heavyweight, titan]
    3: an unusually large enterprise; "Walton built a retail giant"
    4: a very large person; impressive in size or qualities [syn:
       giant, hulk, heavyweight, whale]
    5: someone or something that is abnormally large and powerful
       [syn: giant, goliath, behemoth, monster, colossus]
    6: an imaginary figure of superhuman size and strength; appears
       in folklore and fairy tales
    7: a very bright star of large diameter and low density
       (relative to the Sun) [syn: giant star, giant]