Source : Webster's Revised Unabridged Dictionary (1913)

Halleluiah \Hal`le*lu"iah\, Hallelujah \Hal`le*lu"jah\, n. &
   interj. [Heb. See {Alleluia}.]
   Praise ye Jehovah; praise ye the Lord; -- an exclamation used
   chiefly in songs of praise or thanksgiving to God, and as an
   expression of gratitude or adoration. --Rev. xix. 1 (Rev.
   Ver. )

         So sung they, and the empyrean rung With Hallelujahs.
                                                  --Milton.

         In those days, as St. Jerome tells us,``any one as he
         walked in the fields, might hear the plowman at his
         hallelujahs.''                           --Sharp.

Source : WordNet®

hallelujah
     n : a shout or song of praise to God