From Wikipedia on 7-May-2013
URL: http://en.wikipedia.org/wiki/Paradox
A paradox is an argument that produces an inconsistency, typically within logic or common sense. Most logical paradoxes are known to be invalid arguments but are still valuable in promoting critical thinking. However, some have revealed errors in definitions assumed to be rigorous, and have caused axioms of mathematics and logic to be re-examined (e.g., Russell’s paradox). Still others, such as Curry’s paradox, are not yet resolved. In common usage, the word “paradox” often refers to irony or contradiction. Examples outside logic include the Grandfather paradox from physics, and the Ship of Theseus from philosophy. Paradoxes can also take the form of images or other media. For example, M.C. Escher featured perspective-based paradoxes in many of his drawings.