Nonmonotonic Reasoning (Logic): The term “non-monotonic logic” covers a family of formal frameworks devised to capture and represent defeasible inference — that kind of everyday inference in which reasoners draw conclusions tentatively, reserving the right to retract them in the light of further information. Such inferences are called “non-monotonic” because the set of conclusions warranted on the basis of a given knowledge base does not increase (in fact, it can shrink) with the size of the knowledge-base itself. This is in contrast to classical (first-order) logic, whose inferences, being deductively valid, can never be “undone” by new information. (Stanford Online Encyclopedia)
Source: ‘What is Good? What is Bad? The Value of All Values across Time, Place and Theories’ by John McMurtry, Philosophy and World Problems, Volume I-III, UNESCO in partnership with Encyclopedia of Life Support Systems: Oxford, 2004-11.