A grammatical class of words with limited membership, which have primarily grammatical meaning such as conjunctions (a word conjoining words or phrases or clauses or sentences whose usage increases at 5 years-of-age), demonstratives (a word indicating to which objects a sentence is referring to, with English having four: this, that, these and those), determiners (one of a limited set of noun modifiers determining the referents of noun phrases that are ideal for teaching children labels), pronouns (a word used in place of a noun or noun phrase, with most children using them by 30 months), and auxiliary verbs (a verb that provides further information about a main verb that follows it, but which under certain circumstances can become the main verb as in “I am mending a puncture”). The closed class is considered to be part of the core language, and as such is not expected to change, in contrast to open words. Also known as a function word.
See Grammaticization (or grammaticalization), Language development, Open-class words, Syntax