preposition

In grammar, a preposition is a part of speech that introduces a prepositional phrase. For example, in the sentence "The cat sleeps on the sofa", the word "on" is a preposition, introducing the prepositional phrase "on the sofa". In English, the most oft used prepositions are "of", "to", "in", "for", and "on".
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n. preposition, part of speech that serves to express the relationship between two words (Grammar)

(n.)
A word employed to connect a 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; -- so called because usually placed before the word with which it is phrased; as, a bridge of iron; he comes from town; it is good for food; he escaped by running.
  

(n.)
A proposition; an exposition; a discourse.
  

Noun
1. 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
(hypernym) function word, closed-class word
2. (linguistics) the placing of one linguistic element before another (as placing a modifier before the word it modifies in a sentence or placing an affix before the base to which it is attached)
(hypernym) place, position
(derivation) prepose
(classification) linguistics