1.noun(grammar, strict sense) 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.
2.nounA proposition; an exposition; a discourse.
3.verbTo place in a location before some other event occurs.
"It is important to preposition the material before turning on the machine."