A word such as who or that that makes explicit the relationship among various phrases within a sentence.

a word that is uninflected and serves a grammatical function but has little identifiable meaning

a word that is used to indicate the function of other words, or the relationship between other words in a phrase or sentence, as a preposition or conjunction

A word that carries primarily grammatical, rather than content, meaning.

A word which does not have lexical meaning, which primarily serves to express a grammatical relationship (e.g. AND, OF, OR, THE).

Function word and grammatical word are terms used for CLOSED CLASS words such as PREPOSITION, CONJUNCTION and PRONOUN. See also

a word such as a conjunction, preposition, or article whose primary purpose is to show grammatical relationships in and between sentences; closed-class word; functor; empty word.

Function words (or grammatical words) are words that have little lexical meaning or have ambiguous meaning, but instead serve to express grammatical relationships with other words within a sentence, or specify the attitude or mood of the speaker. Words which are not function words are called content words (or lexical words): these include nouns, verbs, adjectives, and most adverbs, though some adverbs are function words (e.g. then, why). Dictionaries define the specific meanings of content words, but can only describe the general usages of function words.