function word

n. word that indicates mainly grammatical relationship (such as auxiliary verb, preposition or conjunction), form 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. By contrast, grammars describe the use of function words in detail, but have little interest in lexical words.
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Noun
1. a word that is uninflected and serves a grammatical function but has little identifiable meaning
(synonym) closed-class word
(hypernym) word
(hyponym) determiner, determinative
(part-meronym) grammatical meaning