Alleluia is a liturgical hymn sung both during the Divine Liturgy and the Western Rite Mass before the reading of the Gospel. Technically, the word means Praise the Lord.
It is found many times in the book of Psalms. Like Amen or Hosanna, the word has been used in the Church in its original Hebrew.
In the Divine Liturgy as celebrated in the Eastern Church, Alleluia is sung either three times without a refrain (in Greek practice) or nine times with a refrain (in Slavic practice) as a prelude to the reading of the day, just after the apostolic reading at the Divine Liturgy. It is also sung as a funeral dirge, and at most other Orthodox services.
Western Rite Usage
In the Western Rite Mass, the Alleluia occurs at the same place as in the Byzantine liturgy, immediately before the Gospel reading. The Alleluia is chanted straight-through rather than antiphonally but contains a one-verse refrain, something of a cross between Greek and Slavic uses. The Western Rite form begins by chanting the word "Alleluia" twice, followed by a one-verse refrain that varies by the Mass, and concludes by repeating "Alleluia" once again. In Western Rite practice, the Alleluia is not chanted from Septuagesima to Holy Saturday.