The Collaborative International Dictionary of English v.0.48:

Defection \De*fec"tion\, n. [L. defectio: cf. F. d['e]fection.
   See Defect.]
   Act of abandoning a person or cause to which one is bound by
   allegiance or duty, or to which one has attached himself;
   desertion; failure in duty; a falling away; apostasy;
   backsliding. "Defection and falling away from God." --Sir W.
   Raleigh.
   [1913 Webster]

         The general defection of the whole realm. --Sir J.
                                                  Davies.
   [1913 Webster]

WordNet (r) 3.0 (2006):

defection
    n 1: withdrawing support or help despite allegiance or
         responsibility; "his abandonment of his wife and children
         left them penniless" [syn: desertion, abandonment,
         defection]
    2: the state of having rejected your religious beliefs or your
       political party or a cause (often in favor of opposing
       beliefs or causes) [syn: apostasy, renunciation,
       defection]

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "defection":
   abandonment, about-face, accommodation, adaptation, adjustment,
   alienation, alteration, amelioration, apostasy, backsliding,
   bad debt, betrayal, betterment, blemish, bolt, break, breakaway,
   bug, catch, change, change of heart, changeableness,
   constructive change, continuity, conversion, crack, crossing-over,
   default, defect, deficiency, degeneration, degenerative change,
   delinquence, delinquency, dereliction, deserter, desertion,
   deterioration, deviation, difference, disaffection, discontinuity,
   dishonor, dishonoring, disloyalty, disownment, divergence,
   diversification, diversion, diversity, divorce, drawback,
   estrangement, failing, failure, faithlessness, falseness, fault,
   faute, fitting, flaw, flip-flop, foible, forsaking, frailty,
   going over, gradual change, hole, imperfection, improvement,
   inadequacy, infirmity, kink, little problem, melioration,
   mitigation, modification, modulation, nondischarge of debts,
   nonpayment, nonremittal, overthrow, parting, problem, protest,
   protested bill, qualification, radical change, ratting,
   re-creation, realignment, recidivation, recidivism, recreancy,
   redesign, reform, reformation, rejection, remaking, renewal,
   renunciation, repudiation, reshaping, restructuring, reversal,
   revival, revivification, revolution, rift, runout, schism,
   secession, separation, shift, shortcoming, snag, something missing,
   sudden change, switch, taint, tergiversation, total change,
   transition, treason, turn, turnabout, turning traitor,
   uncollectible, upheaval, variation, variety, violent change,
   vulnerable place, walkout, weak link, weak point, weakness,
   worsening