Closed-class words refer to a word such as an article, preposition, or conjunction that plays a secondary role in the meaning of a sentence. Closed-class words is a word from categories, such as determiners/article (examples: a, an, the), auxiliaries (examples, can, would), and prepositions (examples, on, in, over). These categories share the characteristics that they serve grammatical functions, as in articles/determiners mark the beginnings of noun phrases and that speakers cannot readily invent new words to add to these categories in contrast to categories such as noun and verb that readily admit newly coined words. Please see also Function word.
Lexicon of Psychology - Terms, Treatments, Biographies,