de·fea·sance 
(dĭ-fēzəns)
(dĭ-fēzəns)n.
1. A rendering void; an annulment.
2.
a. The voiding of a contract or deed.
b. A clause within a contract or deed providing for annulment.
[Middle English defesaunce, from Anglo-Norman, from Old French defesance, from defesant, present participle of desfaire, to destroy; see DEFEAT.]