delusion, misconception, or mistaken belief, especially one based on invalid or misleading arguments, incomplete knowledge or refusal to accept knowledge, or false notions;
e.g. The notion that ministers never lie is a fallacy.
(noun)
(philosophy, logic) an often plausible argument using false or invalid inference, resulting in a failure of reasoning which renders an argument invalid;
e.g. Identifying logical fallacies dates back to Aristotle and the Greeks: the appeal to ignorance, the slippery slope, or over-simplification.