The Collaborative International Dictionary of English v.0.48:

Compulsion \Com*pul"sion\, n. [L. compulsio. See Compel.]
   The act of compelling, or the state of being compelled; the
   act of driving or urging by force or by physical or moral
   constraint; subjection to force.
   [1913 Webster]

         If reasons were as plentiful as blackberries, I would
         give no man a reason upon compulsion.    --Shak.
   [1913 Webster]

         With what compulsion and laborious flight
         We sunk thus low.                        --Milton.

   Syn: See Constraint.
        [1913 Webster]

WordNet (r) 3.0 (2006):

compulsion
    n 1: an urge to do or say something that might be better left
         undone or unsaid; "he felt a compulsion to babble on about
         the accident" [syn: compulsion, irresistible impulse]
    2: an irrational motive for performing trivial or repetitive
       actions, even against your will; "her compulsion to wash her
       hands repeatedly" [syn: compulsion, obsession]
    3: using force to cause something to occur; "though pressed into
       rugby under compulsion I began to enjoy the game"; "they
       didn't have to use coercion" [syn: compulsion, coercion]

The Devil's Dictionary (1881-1906):

COMPULSION, n.  The eloquence of power.