Exorcism
In Catholic Christianity, exorcisms are performed in the name of Jesus Christ. A distinction is made between a formal exorcism, which can only be conducted by a priest during a baptism or with the permission of a bishop, and "prayers of deliverance" which can be said by anyone.
The Catholic rite for a formal exorcism, called a "Major Exorcism", is given in Section 11 of the Rituale Romanum. The Ritual lists guidelines for conducting an exorcism, and for determining when a formal exorcism is required. Priests are instructed to carefully determine that the nature of the affliction is not actually a psychological or physical illness before proceeding.
Unclean spirit
Such exorcisms are performed rarely by the 21st-century Western church; the more common exorcism involves the ritual preparation of holy water ("aquae"). The 9th-century Stowe Missal preserves an early Celtic formula as "procul ergo hinc, iubente te, domine, omnis spiritus immundus abscedat" ("Therefore at your bidding, Lord, let every unclean spirit depart far from here"). In a Latin version of "The Blessing of the Waters on the Eve of Epiphany" performed in Rome and recorded at the turn of the 19th–20th centuries, the unclean spirit is commanded "per Deum vivum" ("by the Living God"). The modern Latin rite to exorcize holy water banishes any "pestilent spirit" ("spiritus pestilens") or "corrupting atmosphere" ("corrumpens aura"); see Rituals and uses of holy water.