What is "thematic relation"

n. (context linguistics English) Any of various possible semantic relations between a verb and its arguments.

In a number of theories of linguistics, thematic relations is a term used to express the role that a noun phrase plays with respect to the action or state described by a sentence's verb. For example, in the sentence "Susan ate an apple", Susan is the doer of the eating, so she is an agent; the apple is the item that is eaten, so it is a patient. While most modern linguistic theories make reference to such relations in one form or another, the general term, as well as the terms for specific relations, varies; 'participant role', 'semantic role', and 'deep case' have been used analogously to 'thematic relation'.