The Collaborative International Dictionary of English v.0.48:

Interposition \In`ter*po*si"tion\ (?; 277), n. [L. interpositio
   a putting between, insertion, fr. interponere, interpositum:
   cf. F. interposition. See Interpone, Position.]
   [1913 Webster]
   1. The act of interposing, or the state of being interposed;
      a being, placing, or coming between; mediation.
      [1913 Webster]

   2. The thing interposed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

interposition
    n 1: the action of interjecting or interposing an action or
         remark that interrupts [syn: interjection,
         interposition, interpolation, interpellation]
    2: the act or fact of interposing one thing between or among
       others [syn: interposition, intervention]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "interposition":
   encroachment, entrance, entrenchment, impingement, imposition,
   incursion, infiltration, influx, infringement, injection, inroad,
   insinuation, interagency, intercession, interference, interjection,
   interloping, intermediation, interposure, interruption,
   intervention, intrusion, invasion, involvement, irruption,
   mediation, obtrusion, stepping in, trespass, trespassing,
   unlawful entry