Defeasible Reasoning: The forms of reasoning that are not as formal and rigorous as deductive reasoning. The term was brought to prominence by Pollock (1995), reasoning is defeasible when the corresponding argument, though not valid, is yet rationally meritorious. Pollock developed the term to help work through issues in epistemology. Given the relationship between epistemology and logic, it is not surprising that the term has found employment in logic. In recent work, the term has typically been limited to inferences involving rough-and-ready, exception-permitting generalizations, that is, inferring what has or will happen on the basis of what normally happens (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.