any of the classes into which words in some languages, as Latin and English, have traditionally been divided on the basis of their meaning, form, or syntactic function, as, in English, noun, pronoun, verb, adverb, adjective, preposition, conjunction, and interjection.
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British Dictionary definitions for part of speech
a class of words sharing important syntactic or semantic features; a group of words in a language that may occur in similar positions or fulfil similar functions in a sentence. The chief parts of speech in English are noun, pronoun, adjective, determiner, adverb, verb, preposition, conjunction, and interjection: Abbreviation: POS
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012