A function word that combines with a noun or pronoun or noun phrase to form a prepositional phrase that can have an adverbial or adjectival relation to some other word WordNet 3.0 & Open English WordNet

A word or phrase able to connect a following noun or noun phrase (and often other parts of the speech) as a complement to some other part of the sentence, expressing a relation between them. OmegaWiki

Any of a class of non-inflecting words typically employed to connect a following noun or a pronoun, in an adjectival or adverbial sense, with some other word: a particle used with a noun or pronoun (in English always in the objective case) to make a phrase limiting some other word. Wiktionary