In certain theories of linguistics, thematic relations, also known as semantic roles or thematic roles, are the various roles that a noun phrase may play with respect to the action or state described by a governing verb, commonly the sentence's main verb. For example, in the sentence "Susan ate an apple", Susan is the doer of the eating, so she is an agent; an apple is the item that is eaten, so it is a patient.
Since their introduction in the mid-1960s by Jeffrey Gruber and Charles Fillmore, semantic roles have been a core linguistic concept and ground of debate between linguist approaches, because of their potential in explaining the relationship between syntax and semantics (also known as the syntax-semantics interface)-- that is, how meaning affects the surface syntactic codification of language. The notion of semantic roles plays a central role especially in functionalist and language-comparative (typological) theories of language and grammar.