Open and closed class

A set of things being closed refers to it being a static group, one that doesn't continually acquire words, or at best a lot more slowly, which also means you could make a reasonably complete list of these.

In many languages, closed classes include determiners, prepositions (adpositions in general), pronouns, conjunctions, and particles.

An open word class is a category of that may grow, through creativity, borrowing, coining, compounding, derivation, and other word formation.

Nouns, verbs and adjectives are probably always open, and interjections often are as well.

Auxiliary verbs may have a common core that is closed, but is otherwise relatively open(verify).

Notes:

• 'class' because this historically focused first on word class

• You will find explanations focusing on whether a specific lexical category / word class is open or closed, but it could apply to anything you might list, e.g. specific types of MWEs.

in which case we're not talking about just words anymore

• Openness versus closedness regularly comes up in controlled corpora and computational linguistics.

For example, WordNet does not contain certain small closed classes, apparently because they're expected to be known to you.