In the context of linguistic morphology, an uninflected word is a word that has no morphological marks (inflection) such as affixes, Umlaut, Ablaut, consonant gradation, etc., indicating declension or conjugation.

In English and many other languages, uninflected words include prepositions, interjections and conjunctions, often called invariable words. These cannot be inflected under any circumstances (unless they are used as different parts of speech, as in "ifs and buts", or "humming and hawing").

Only words which cannot be inflected at all should be called "uninflected" or "invariable", but in common usage these terms can be extended to words that appear in their basic form. For example, English nouns are said to be uninflected in the singular, while they show inflection in the plural (represented by the affix -s/-es).

In the strict sense, only English mass nouns (such as sand, information, or equipment) are truly uninflected, since they have only one form that does not change; count nouns are always inflected for number, even if the singular inflection is shown by an "invisible" affix (the null morpheme). In the same way, English verbs are inflected for person and tense even if the morphology showing those categories is realized as null morphemes. In contrast, other analytic languages like Mandarin Chinese have true uninflected nouns and verbs, where the notions of number and tense are completely absent. The term "uninflected" can also be restricted to one or more morphological features; for example, one can say that Japanese verbs are uninflected for person and number, but they do inflect for tense, politeness, and several moods and aspects.

In many inflected languages, such as Greek and Russian, some nouns and adjectives of foreign origin are left uninflected in contexts where native words would be inflected; for instance, the name Abraam in Greek (from Hebrew), the modern Greek word mple (French bleu), the Italian word computer, and the Russian words kenguru (kangaroo) and pal'to (coat).