Grammatical relations (GRs) are structurally defined relations between words in phrases and clauses. Common terms used to refer to particular grammatical relations are subject, direct object, indirect object, ergative, absolutive, genitive, and oblique. Sometimes the oblique relation (discussed below) is considered to be the absence of a grammatical relation. Like other structural notions, GRs are defined independently of function (such as semantics or topicality), though they clearly have communicative functions. Even as the structure of any tool is logically distinct from (though intimately connected to) its function, so GRs are logically distinct from the functions that they perform. Nevertheless, it is important to recognize that GRs play a significant role in expressing meaningful distinctions, such as who is acting upon whom, what is topical, and so on.

A second important fact about GRs is that they are essentially relational concepts. In other words, they don't exist unless there are two elements that are related. A nominal element by itself does not “have” a grammatical relation. It is only when it occurs in a structure with a verb that we can say that it is a “subject” or an “object,” etc. In fact, it may be better to always say “subject of” or “object of” since these terms make it clear that there must be another element in the construction. The grammatical properties that identify GRs are determined by syntactic constructions, and not simply by semantic properties of individual nouns or verbs.