1rare The action or an act of pouring a liquid on to something or someone.
2historical The pouring of water on the body or a part of the body for therapeutic purposes; an instance of this. Frequently with distinguishing word specifying the temperature of the water, especially in "cold affusion". Also figurative.
Affusion of cold water was used commonly in the treatment of fevers until the early 20th century.
The administration of baptism by pouring or sprinkling water over a person's head (opposed to immersion and aspersion); an instance of this.
Now the most usual form of baptism in the Roman Catholic and Anglican Churches.
Early 17th century (in an earlier sense). From (i) French affusion parenchyma;.