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.
Different languages may have different lexical categories, or they might associate different properties to the same one. For example, Japanese has as many as three classes of adjectives where English has one; Chinese, Korean and Japanese have classifiers while European languages do not grammaticalize these units of measurement (a pair of pants, a grain of rice); many languages don't have a distinction between adjectives and adverbs, adjectives and verbs (see stative verbs) or adjectives and nouns, etc. Some argue that the formal distinctions between parts of speech must be made within the framework of a specific language or language family, and should not be carried over to other languages or language families.
The classification of words into lexical categories is found from the earliest moments in the history of linguistics. In the Nirukta, written in the 5th or 6th century BC, the Sanskrit grammarian Yāska defined four main categories of words:
These four were grouped into two large classes: inflected (nouns and verbs) and uninflected (pre-verbs and particles).
The Tamil grammarian Tolkappian in his work Tolkappiyam dated variously between 1st CE and 10th CE, classifies words in Tamil as
A century or two later, the Greek scholar Plato wrote in the Cratylus dialog that "... sentences are, I conceive, a combination of verbs [rhēma] and nouns [ónoma]". Another class, "conjunctions" (covering conjunctions, pronouns, and the article), was later added by Aristotle.
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