In linguistics, an open class (or open word class) is a word class that accepts the addition of new items, through such processes as compounding, derivation, coining, borrowing, etc. Typical open word classes are nouns, verbs and adjectives.

Open-class words are not considered part of the core language and as such they can be changed, replaced or dropped from the common lexicon, which can encompass many thousands of them. For living languages, this change is noticeable within an individual lifespan, and usually faster. Closed-class words, on the other hand, are always relatively few and resistant to change. (For example, most English-speaking people employ more or less the same prepositions and pronouns as their great-grandparents, but different and probably more nouns and verbs.)