In grammar, a part of speech or part-of-speech (abbreviated as POS or PoS, also known as word class or grammatical category) is a category of words (or, more generally, of lexical items) that have similar grammatical properties. Words that are assigned to the same part of speech generally display similar syntactic behavior (they play similar roles within the grammatical structure of sentences), sometimes similar morphological behavior in that they undergo inflection for similar properties and even similar semantic behavior. Commonly listed English parts of speech are noun, verb, adjective, adverb, pronoun, preposition, conjunction, interjection, numeral, article, and determiner.

Other terms than part of speech—particularly in modern linguistic classifications, which often make more precise distinctions than the traditional scheme does—include word class, lexical class, and lexical category. Some authors restrict the term lexical category to refer only to a particular type of syntactic category; for them the term excludes those parts of speech that are considered to be function words, such as pronouns. The term form class is also used, although this has various conflicting definitions. Word classes may be classified as open or closed: open classes (typically including nouns, verbs and adjectives) acquire new members constantly, while closed classes (such as pronouns and conjunctions) acquire new members infrequently, if at all.

Almost all languages have the word classes noun and verb, but beyond these two there are significant variations among different languages. For example:

Because of such variation in the number of categories and their identifying properties, analysis of parts of speech must be done for each individual language. Nevertheless, the labels for each category are assigned on the basis of universal criteria.

The classification of words into lexical categories is found from the earliest moments in the history of linguistics.

In the Nirukta, written in the 6th or 5th century BCE, the Sanskrit grammarian Yāska defined four main categories of words:

These four were grouped into two larger classes: inflectable (nouns and verbs) and uninflectable (pre-verbs and particles).

The ancient work on the grammar of the Tamil language, Tolkāppiyam, argued to have been written around 2nd century CE, classifies Tamil words as peyar (பெயர்; noun), vinai (வினை; verb), idai (part of speech which modifies the relationships between verbs and nouns), and uri (word that further qualifies a noun or verb).