form class

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A set of words that have one or more grammatical or syntactic characteristics in common, such as the class of transitive verbs in English.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun taxonomy A collection of organisms that is given formal recognition at the rank of class with a taxonomic name, but which is known to be an artificial group rather than a natural one.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun one of the traditional categories of words intended to reflect their functions in a grammatical context