Patient is a semantic role that is usually the surface object of the verb in a sentence.
Some linguists define the patient and affected semantic roles in slightly different ways.
Longacre 1983 155–156 (following Chafe) defines a patient as the entity:
Larson 1984 199–203 defines the affected role as the:
(Longacre 1983)
The entity predicated with a state or location:
The entity undergoing a change of state or location:
He opened the door.
He threw the ball across the yard.
The entity which is possessed, acquired, or exchanged:
John has a new book.
John bought a new book.
John gave Mary a new book
(Larson 1984)
Affected semantic role
The thing that is affected by an event:
The dog ate the meat.
The tree fell on the house.
The person or thing that undergoes a process:
The person who experiences an event:
Longacre, Robert E. 1983.The grammar of discourse. New York: Plenum.
Larson, Mildred L. 1984.Meaning-based translation: A guide to cross-language equivalence. Lanham, MD: University Press of America.