What is a semantic role?

Semantic case, thematic role, theta role (generative grammar), and deep case (case grammar)

If, in some real or imagined situation, someone named John purposely hits someone named Bill, then John is the agent and Bill is the patient of the hitting event. Therefore, the semantic role of Bill is the same (patient) in both of the following sentences:

• John hit Bill.
• Bill was hit by John.

In both of the above sentences, John has the semantic role of agent.

The semantic roles most often embodied by the grammatical relations of subject, object and indirect object in natural languages are: agent, force, instrument, experiencer, recipient, and patient. Other semantic roles are more likely to be embodied in oblique (adpositional) phrases or adverbials.