Eucharistic adoration is a time for prayer in which Christians—particularly the Catholic faithful—adore our Lord Jesus Christ in the Most Blessed Sacrament. Adoration can take place either when the Church reserves our Eucharistic Lord Jesus in the tabernacle, or when a priest or deacon exposes the consecrated Host in a monstrance outside of Mass.
Origins
Eucharist adoration began at the Last Supper, when Jesus instituted the holy sacrifice of the Mass, and the apostles revered and received him in the Most Blessed Sacrament. In the early Church, the Church affirmed the Real Presence, including by reserving the Eucharist for the sick and dying.
Adoration
Adoration of the Blessed Sacrament, also known as adoration of the Eucharist or adoration of the Holy Eucharist, is an act of worship and thus given to God alone. In adoration, Catholics worship and glorify Jesus Christ who is truly present in the Eucharist. Worship (latria) differs markedly from the veneration and honor (dulia and hyperdulia) given to the saints, especially the Blessed Mother.
During exposition of the Blessed Sacrament, the consecrated Host is placed in a monstrance and set on the altar by either a priest or deacon. Traditionally, a hymn is sung at the beginning, typically O Salutaris Hostia, which St. Thomas Aquinas wrote at the behest of Urban IV for the first universal celebration of Corpus Christi in 1264
Benediction
After the time of adoration comes benediction, when the priest or deacon, and also the faithful, join in singing a hymn and offer other prayers. During benediction, those gathered traditionally sing Tantum Ergo, another hymn which Urban IV had St. Thomas Aquinas write for the celebration of Corpus Christi. After the hymn, the faithful remain kneeling while the priest or deacon incenses the Eucharist if exposition has taken place with a monstrance. After this, the ordained minister will raise the monstrance and bless the faithful gathered with the Blessed Sacrament, making the sign of the cross over them. Then the congregation recites the Divine Praises: