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]

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "compulsion":
   abstraction, abulia, alienation, amperage, anxiety,
   anxiety equivalent, anxiety state, apathy, armipotence, authority,
   beef, black power, brute force, catatonic stupor, charge, charisma,
   clout, coercion, cogence, cogency, complex, compulsoriness,
   constraint, dejection, depression, detachment, dint, drive,
   driving, driving force, duress, effect, effectiveness,
   effectuality, elation, emotionalism, energy, euphoria, exigency,
   fascination, fixation, fixed idea, flower power, folie du doute,
   force, force majeure, forcefulness, full blast, full force,
   hang-up, hypercathexis, hypochondria, hysteria, hysterics,
   idee fixe, impellent, impelling, impelling force, impetus, impulse,
   impulsion, incentive, incitement, indifference, influence,
   insensibility, irresistible force, irresistible impulse, lethargy,
   main force, main strength, mana, mandatedness, mandatoriness,
   mania, melancholia, mental distress, might, might and main,
   mightiness, moment, momentum, monomania, morbid drive,
   motive power, moxie, muscle power, necessariness, necessitation,
   necessitude, necessity, need, obligation, obligatoriness,
   obligement, obsession, obsessive compulsion,
   pathological indecisiveness, pinch, pizzazz, poop, possession,
   potence, potency, potentiality, power, power pack, power structure,
   power struggle, powerfulness, preoccupation, prepossession,
   prepotency, press, pressing, pressure, productiveness,
   productivity, propulsion, psychalgia, psychomotor disturbance,
   puissance, pull, punch, push, rash impulse, ruling passion, sinew,
   steam, strength, stress, strong arm, stupor, superiority,
   superpower, thrust, tic, twitching, unresponsiveness, urge,
   urgency, validity, vehemence, vigor, vim, violence, virility,
   virtue, virulence, vitality, wattage, weight, withdrawal



The Devil's Dictionary (1881-1906):

COMPULSION, n.  The eloquence of power.