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]