From Teflpedia

The agent (/eɪʤʌnt/) is the thematic relation of the cause or initiator to an event and thereby possesses agency[1]

  • In the active sentence "the dog bit the man", the agent is the dog, and the dog is also the subject
  • In the passive sentence "the man was bitten by the dog", describing the same event, "the dog" is still the agent, even though now "the man" is the subject.

Compare patient.