What does alleluia or alleluiah mean?
The definition of alleluia and alleluiah
Al`le*lu”ia, Al`le*lu”iah, n.
Etym: [L. alleluia, Gr. hall-yah. See Hallelujah.]

An exclamation signifying Praise ye Jehovah. Hence: A song of praise to God. See Hallelujah, the commoner form. I heard a great voice of much people in heaven, saying, Alleluia. Rev. xix. 1.