term:

Defeasance

defeasance n

[Anglo-French defesance, literally, undoing, destruction, from Old French deffesant, present participle of deffaire to destroy, undo see defeat ]
1 a : a condition (as in a deed or will) that upon fulfillment terminates a property interest
b : an instrument setting forth such a condition
2 : a rendering null or void

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.