The Collaborative International Dictionary of English v.0.48:

Cogency \Co"gen*cy\, n. [See Cogent.]
   The quality of being cogent; power of compelling conviction;
   conclusiveness; force.
   [1913 Webster]

         An antecedent argument of extreme cogency. --J. H.
                                                  Newman.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cogency
    n 1: persuasive relevance
    2: the quality of being valid and rigorous [syn: cogency,
       validity, rigor, rigour]

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "cogency":
   acuity, acumen, acuteness, advantageousness, agreeableness,
   amperage, apperception, armipotence, astuteness, auspiciousness,
   authoritativeness, authority, bearing, beef, beneficialness,
   benevolence, benignity, black power, brute force, charge, charisma,
   class, clout, cogence, compulsion, concern, connection, desert,
   dint, discernment, drive, duress, effect, effectiveness,
   effectuality, energy, excellence, expedience, fairness,
   farseeingness, farsightedness, favorableness, fineness,
   first-rateness, flower power, force, force majeure, forcefulness,
   foresight, foresightedness, full blast, full force, good reason,
   good reasoning, goodliness, goodness, grace, healthiness,
   helpfulness, incisiveness, influence, insight, ironclad reasoning,
   justness, kindness, knockdown argument, longheadedness,
   longsightedness, main force, main strength, mana, merit, might,
   might and main, mightiness, moxie, muscle power, niceness,
   penetration, perception, perceptiveness, percipience,
   perspicaciousness, perspicacity, perspicuity, perspicuousness,
   pertinence, pizzazz, pleasantness, poop, potence, potency,
   potentiality, power, power pack, power structure, power struggle,
   powerfulness, prepotency, productiveness, productivity,
   profitableness, providence, puissance, pull, punch, push, quality,
   relevance, rewardingness, right thinking, sagaciousness, sagacity,
   sensibility, sinew, skillfulness, solidity, sound evidence,
   sound reasoning, soundness, steam, strength, strong argument,
   strong arm, strong point, substantiality, superiority, superpower,
   trenchancy, usefulness, validity, value, vehemence, vigor, vim,
   virility, virtue, virtuousness, virulence, vitality, wattage,
   weight, wholeness, worth