| Definition -
A noun that labels the person or thing that is the source of action. As opposed to a recipient noun, which labels the person or thing that is receiving the action.
Note: The suffixes -er, -ist, and -or are used to form agent nouns.
(1) adviser (someone who advises)
(2) evangelist (someone who evangelizes)
(3) guarantor (someone who gives a guaranty)
Oxford English Dictionary - The term's first citation is from 1879:
"There is hardly a suffix by which action-nouns are formed which does not also make agent-nouns or adjectives."
( Whitney Sanskr. Gram. 374)