"Hallelujah" is a Hebrew word meaning "praise the LORD," and in the King James Bible it appears in the Greek spelling "Alleluia" — four times in Revelation 19.
Revelation 19:1 in the King James Version reads:
And after these things I heard a great voice of much people in heaven, saying, Alleluia; Salvation, and glory, and honour, and power, unto the Lord our God:
The verse opens Revelation 19, a chapter in which the word appears repeatedly in a scene of heavenly praise. Read it in context at Revelation 19:1, or the full chapter at Revelation 19. Encyclopedia entry: en.wikipedia.org/wiki/Hallelujah.