invariable

Invariable is an adjective with two principal senses. In ordinary use it means something that does not change or remains fixed. In linguistic terminology, invariable refers to a word that does not alter its form to reflect grammatical categories such as tense, number, gender, case, or mood. In this sense the term is closely related to uninflected.

In grammar, invariable words belong to classes that typically do not undergo morphological change across contexts. Common examples include many prepositions (in, on, at), conjunctions (and, but, or), and a broad range of adverbs (very, quickly). Determiners such as the definite article the are often treated as invariable in form in English, though some languages distinguish forms for number or other categories. By contrast, verbs and adjectives are usually inflected and thus variable in their forms.

The concept is most widely used in traditional and descriptive grammars, especially for languages with rich inflection. Some modern grammars prefer terms like uninflected or non-inflecting to describe this phenomenon, focusing on morphological behavior rather than a fixed part of speech. The category helps explain how certain function words resist change even as other words around them seize different grammatical roles.

See also: Inflection, Uninflected, Parts of speech.