function word

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A word, such as a preposition, a conjunction, or an article, that has little semantic content of its own and chiefly indicates a grammatical relationship.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun linguistics 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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