"Praise ye Jehovah, frequently rendered "Praise ye the LORD," "stands at the beginning of ten of the psalms (106, 111-113, 135," "146-150), hence called "hallelujah psalms." From its frequent" occurrence it grew into a formula of praise. The Greek form of "the word (alleluia) is found in Rev. 19:1, 3, 4, 6."

See where Hallelujah occurs in the Bible...

Definition of Hallelujah:
"praise the Lord"