The Collaborative International Dictionary of English v.0.48:

Preposition \Prep`o*si"tion\, n. [L. praepositio, fr. praeponere
   to place before; prae before + ponere to put, place: cf. F.
   pr['e]position. See Position, and cf. Provost.]
   [1913 Webster]
   1. (Gram.) 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.
      [1913 Webster]

   2. A proposition; an exposition; a discourse. [Obs.]
      [1913 Webster]

            He made a long preposition and oration. --Fabyan.
      [1913 Webster]

WordNet (r) 3.0 (2006):

preposition
    n 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
    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)