The Collaborative International Dictionary of English v.0.48:

Juxtaposition \Jux`ta*po*si"tion\
   (j[u^]ks`t[.a]*p[-o]*z[i^]sh"[u^]n), n. [L. juxta near +
   positio position: cf. F. juxtaposition. See Just, v. i.,
   and Position.]
   A placing or being placed in nearness or contiguity, or side
   by side; as, a juxtaposition of words.
   [1913 Webster]

         Parts that are united by a a mere juxtaposition.
                                                  --Glanvill.
   [1913 Webster]

         Juxtaposition is a very unsafe criterion of continuity.
                                                  --Hare.
   [1913 Webster]

WordNet (r) 3.0 (2006):

juxtaposition
    n 1: the act of positioning close together (or side by side);
         "it is the result of the juxtaposition of contrasting
         colors" [syn: juxtaposition, apposition, collocation]
    2: a side-by-side position

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "juxtaposition":
   abutment, abuttal, accession, addition, adjacency, adjoiningness,
   adjunct, adjunction, affixation, agglutination, annexation,
   apposition, appulse, assemblage, assembly, attachment,
   augmentation, call-up, canvass, census, collection, colligation,
   collocation, combination, comparison, concourse, concurrence,
   confluence, conflux, congregation, conjugation, conjunction,
   connection, conterminousness, contiguity, convergence, corralling,
   coterminousness, data-gathering, gathering, increase, ingathering,
   inventory, joining, junction, mobilization, muster, perigee,
   perihelion, prefixation, reinforcement, rodeo, roundup,
   suffixation, superaddition, superfetation, superjunction,
   superposition, supplementation, survey, syzygy, union, uniting