In linguistics, declension is a feature of inflected languages: the changing of a noun to indicate its grammatical role. This is seen, for example, in Latin. In English grammar, the same task is now accomplished with word order, though a few remnants of an older declined form of English still exist (e.g. the words "who" and "whom").

In inflected languages, nouns are said to decline into different forms, or morphological cases. Morphological cases are one way of indicating grammatical case; other ways are listed below.

Languages are categorized into several case systems, based on how they group verb agents and patients into cases:

  • Nominative-accusative: The agent of a verb is always in the nominative case. The patient, if one is specified, is in the accusative case.
  • Ergative-absolutive: The patient of a verb is always in the absolutive case. The agent, if one is specified, is in the ergative case.
  • Active: The agent of a verb is always in the subject case, and the patient is always in the object case. The case does not depend on whether a verb is used in a transitive or intransitive form.
  • Trigger: One noun in a sentence is the topic or focus. This noun is in the trigger case, and information elsewhere in the sentence (e.g. a verb affix in Tagalog) specifies the role of the trigger. The trigger may be identified as the agent, patient, etc. Other nouns may be inflected for case, but the inflections are overloaded; for example, in Tagalog, the subject and object of a verb are both expressed in the genitive case when they are not in the trigger case.
The following are systems that some languages use to mark case instead of, or in addition to, declension:
  • Positional: Nouns are not inflected for case; the position of a noun in the sentence expresses its case.
  • Prepositional/postpositional: Nouns are accompanied by words that mark case, but the noun itself is not modified.

See Nominative case, Accusative case, Dative case, Ergative case, Absolutive case, Genitive case, Vocative case, Partitive case, Inessive case, Elative case, Illative case, Adessive case, Allative case, Ablative case, Essive case, Translative case, Instructive case, Abessive case, Comitative case, Prolative case, Locative case, Possessive case, Instrumental case.

Some languages have more than 20 cases. For an example of a language that uses a large number of cases, view the "Cases" section in the Finnish language grammar article.

Some languages have different declension for different classes of nouns, e.g. persons, animals, things.