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In grammar, a part of speech (also a word class, a lexical class, or a lexical category) is a linguistic category of words (or more precisely lexical items), which is generally defined by the syntactic or morphological behaviour of the lexical item in question. Common linguistic categories include noun and verb, among others. There are open word classes, which constantly acquire new members, and closed word classes, which acquire new members infrequently if at all.
Almost all languages have the lexical categories noun and verb, but beyond these there are significant variations in different languages.[1] For example, Japanese has as many as three classes of adjectives where English has one; Chinese, Korean and Japanese have nominal classifiers whereas European languages do not; many languages do not have a distinction between adjectives and adverbs, adjectives and verbs (see stative verbs) or adjectives and nouns[citation needed], etc. This variation in the number of categories and their identifying properties entails that analysis be done for each individual language. Nevertheless the labels for each category are assigned on the basis of universal criteria.[1]

The classification of words into lexical categories is found from the earliest moments in the history of linguistics.[2] In the Nirukta, written in the 5th or 6th century BC, the Sanskrit grammarian Yāska defined four main categories of words:[3] nāma – nouns or substantives ākhyāta – verbs upasarga – pre-verbs or prefixes nipāta – particles, invariant words (perhaps prepositions)
These four were grouped into two large classes: inflected (nouns and verbs) and uninflected (pre-verbs and particles).
The ancient work on the grammar of the Tamil language, Tolkappiyam, dated variously between 1st and 10th centuries AD, classifies words[4] in Tamil as peyar (noun), vinai (verb), idai (part of speech which modifies the relationships between verbs and nouns) and uri (word that