Alleluia (Hallelujah) is a Hebrew imperative meaning "praise the Lord," which entered Christian worship as a word of joy carried over untranslated from the Hebrew Psalms, where it opens and closes a number of the songs of praise. In Christian usage it became an exclamation of praise in its own right, sung especially in the Easter season and attached to the reading of the Gospel and to psalmody, and the Book of Revelation places it in the mouths of the heavenly host. Its untranslated survival, like the Amen and the Hosanna, links Christian worship directly to its Hebrew and synagogue roots, the church continuing to praise God in the very word of the psalmist. In the Western tradition the Alleluia is notably suppressed during the penitential season of Lent and restored with emphasis at Easter, so that its absence and return themselves mark the rhythm of the liturgical year.
The Hallel psalms, Psalms 113 to 118, are the primary biblical source of Hebrew hallelujah, sung at festivals in Jewish liturgy, and it is this psalmic collection that early Christian communities inherited directly when the word passed untranslated into Greek and then Latin. Its jubilus, the extended melisma stretching the final vowel of the sung Alleluia in Gregorian chant, became a specific subject of medieval music theory and is credited by some liturgical historians as a spur to the later development of sequence hymns, composed texts fitted to an existing melisma's notes so the wordless elaboration could carry sung words. Its total suppression through Lent and dramatic return at the Easter Vigil makes its absence and reappearance one of the most structurally deliberate uses of a single word in Christian practice.