Behind The Art - Alleluia:

The Hebrew word hallel means “praise”, and Yah or Jah is shorthand for “Jehovah,” the Lord. So, “Alleluia” is derived from “Praise the Lord”. In the book of Revelation, a heavenly multitude shouts, “Alleluia! For the Lord God Omnipotent reigns! Let us be glad and rejoice and give Him glory.” We use the word as an exclamation of praise, joy, thanks. Hearing its overwhelming repetition in the choruses of Handel’s Messiah is probably one of the reasons I see it that way, too.