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)

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "preposition":
   adjectival, adjective, adverb, adverbial, adversative conjunction,
   attributive, conjunction, conjunctive adverb,
   coordinating conjunction, copulative, copulative conjunction,
   correlative conjunction, disjunctive, disjunctive conjunction,
   exclamatory noun, form class, form word, function class, gerundive,
   interjection, part of speech, participle, particle,
   past participle, perfect participle, present participle,
   subordinating conjunction, verbal adjective