1. One of a group of traditional classifications of words according to their functions in context, including the noun, pronoun, verb, adjective, adverb, preposition, conjunction, and interjection, and sometimes the article.
2. A word considered as a part of speech.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.