Defeasibility is the property of something – such as a contract, a proposition or an understanding – that can be annulled, invalidated, or similarly "defeated". In law, it refers to the possibility of a contract or other legal agreement being terminated by circumstances that arise later, or of legal reasoning being overturned. In philosophy – especially in epistemology, ethics, or the philosophy of law – it refers to the possibility of a particular principle, rule or understanding being overridden in appropriate circumstances.
In pragmatics, a subfield of linguistics, it refers to the fact that certain kinds of implicitly conveyed information such as conversational implicatures and presuppositions can be "defeated" without sounding contradictory. For example, "This flag is green" normally implicates that it is completely green; but "This flag is green. It contains a red disc" is no contradiction. Likewise, "John never stopped smoking" normally presupposes that he used to smoke (and still does), but you can say "John never stopped smoking because actually he never did smoke" without contradiction.