A word that is uninflected and serves a grammatical function but has little identifiable meaning WordNet 3.0 & Open English WordNet

In linguistics, function words are words that have little lexical meaning or have ambiguous meaning and express grammatical relationships among other words within a sentence, or specify the attitude or mood of the speaker. Wikipedia

Words with little/ambiguous lexical meaning that express grammatical relationships or specify the speaker’s attitude/mood of the speaker; e.g. articles, pronouns, adpositions, conjunctions, auxiliary verbs, interjections, particles Wikidata

A word that has little meaning when standing alone, but instead serves to express grammatical relationships with other words within a sentence, or specify the attitude or mood of the speaker, such as prepositions, pronouns, auxiliary verbs, or conjunctions. Wiktionary

Function word. Wiktionary (translation)