Declension occurs in many of the world's languages. Among modern languages, declension is an important aspect of Arabic, Finnish, Turkish, Hungarian, many Amerindian languages such as Quechua, Bantu languages such as Zulu, Slavic languages such as Russian and Ukrainian, some Germanic languages such as High German, and some others. In the ancient world, languages that were highly declined included Sanskrit, Greek, Latin, and Hebrew. Old English was a moderately inflected language, as befits its Indo-European and especially its Germanic linguistic ancestry, but its declensions greatly simplified as it evolved into Modern English.
Declensions may apply to nouns, pronouns, adjectives, adverbs, numerals, and articles to indicate number (at least singular and plural), case (nominative or subjective, genitive or possessive, etc.), and/or gender. A declension is also a group of nouns that follow a particular pattern of inflection.
In the professional jargon specific to the language sciences, the term "declension" is sometimes used in a humorous way, to indicate where the meaning of an expression is off from the standard form; for example, comedy which isn't funny could be said to have "humor declension."
In Modern English, the system of declensions is very simple compared to some other languages, so much so that the term declension is rarely applied to English in practice. Most nouns in English have distinct singular and plural forms and have distinct plain and possessive forms. Plurality is most commonly shown by the clitic -s (or -es), whereas possession is always shown by the clitic -'s (or by just the apostrophe for most plural forms ending in s) attached to the noun. Consider, for example, the forms of the noun girl:
Most speakers pronounce all of the forms other than the singular plain form (girl) exactly the same (though the elided possessive-indicating s of the plural possessive may be realised as in some speakers' pronunciations, being separated from the plural-indicating s normally by a central vowel such as ). By contrast, a few nouns are slightly more complex in their forms. For example:
In that example, all four forms are pronounced in a distinct manner.
There can be other derivations from nouns that are not usually considered declensions. For example, the proper noun Britain has the associated descriptive adjective British and the demonym Briton. Though these words are clearly related and are generally considered cognates, they are not specifically treated as forms of the same word and thus not declensions.