noun
: a class of linguistic forms that can be used in the same position in a construction and that have one or more morphological or syntactical features in common
book, hat, going, deceased, little one, and rapidly flowing stream belong to the same form class as shown by the fact that each can be used in the same position in the construction the — is
books and hats belong to the form classes of nouns and of plurals
opened and walked belong to the form classes of verbs and of past tenses
— see major form class