Allēlouia (G239) is a transliteration of the Hebrew Hallelu-Yah (Praise YHWH!). It appears 4 times in the NT — all in Revelation 19, in the heavenly worship scene. The word combines hallelu (imperative plural of halal: to praise) and Yah (short form of YHWH). The Hallel psalms (Psalm 113–118) frame the Passover liturgy with hallelujah.
Allēlouia in Revelation 19 is the eschatological shout of the redeemed. Four times in verses 1–6, the heavenly multitude cries allēlouia over the judgment of Babylon and the arrival of the wedding supper. This is the final vindication of God's people — history ends in Hallelujah. Jesus likely sang these words at the Last Supper (Matthew 26:30 — 'when they had sung a hymn,' the Hallel). The cross and the victory are both framed with praise.