part of speech, in traditional English grammar, any one of about eight major classes of words, based on the parts of speech of ancient Greek and Latin. The parts of speech are noun, verb, adjective, adverb, interjection, preposition, conjunction, and pronoun. Some grammarians add articles, quantifiers, and numerals. These word classes have traditional definitions in grammar books, i.e., "a noun is the name of a person, place, or thing" without reference to grammatical function. By this strict definition the word toy would be a noun in the sentence "The toy is under the tree" and in the sentence "It is a toy dog." However, an alternate method of defining parts of speech is in terms of the structural features and distribution patterns within a sentence. Thus toy would constitute a different part of speech in each of the above sentences since the word functions in different environments in each sentence, i.e., as a subject and as a modifier. Some English parts of speech (nouns, verbs, etc.) are productive classes allowing new members; others, with functional rather than lexical meaning (prepositions, articles, conjunctions) are nonproductive, having a limited number of members. See also inflection.
See L. Bloomfield, Language (1933); C. Fries, The Structure of English (1952); W. N. Francis, The Structure of American English (1958); O. Jespersen, The Philosophy of Grammar (1965); F. R. Palmer, Grammar (1971); C. L. Baker, English Syntax (1989).