In its most general sense, the term refers to a network of patterned relationships constituting the organization of language. Language as a whole is then characterized as a system (cf. the ‘linguistic system of English’, etc.) - and often as a hierarchically ordered arrangement of systems. In one view, the ‘language system’ is constituted by the phonological, grammatical and semantic systems; the phonological system comprises the segmental and suprasegmental systems; the segmental system comprises the vowel and consonant systems; and so on.

Within this totality, the term ‘system’ may be applied to any finite set of formally or semantically connected units, where the interrelationships are mutually exclusive (i.e. two members of the same system cannot co-occur) and mutually defining (i.e. the meaning of one member is specifiable only with reference to others). Other ‘grammatical systems’ would include determiner / tense / mood / prepositional / negation, etc. The term would not normally be applied to open-class items, such as nouns, adjectives, sentences, etc., unless it meant the set of formal grammatical relationships subsumed under that heading, e.g. the ‘noun system’ would mean the set of syntagmatic and paradigmatic relationships which define the class of nouns.

In Hallidayan linguistics, the notion of system receives a special status. In scale-and-category grammar, it is one of the four central categories recognized by the theory (the others being unit, structure and class): ‘systems’ are finite sets of paradigmatically related items functioning in classes. In the later development of this approach, systemic grammar, the notion of system is made a central explanatory principle, the whole of language being conceived as a ‘system of systems’. Systemic grammar is concerned to establish a network of systems of relationships, in the above sense, which will account for all the semantically relevant choices in the language as a whole.

The adjective systematic is often used in linguistics in its everyday sense, but in certain contexts (usually in relation to phonetics and phonology) it receives a restricted definition. In generative grammar, it has been used to refer to two levels of representation in the phonological component of the grammar: systematic phonemic and systematic phonetic levels are distinguished, the implication being that the terms of these analyses are being seen as in systemic correspondence with other aspects of the grammar (e.g. the morphological relationships between items).