a set of words that display the same formal (linguistic) properties, especially their inflections and distribution.
Word class (similar to part of speech) is a set of words that display the same formal properties, especially their inflections and distribution.
Types:
1) lexical (or open or form) classes (nouns, verbs, adjectives, adverbs) 2) function (or closed or structure) classes (determiners, particles, prepositions etc.).