Pronunciation: (fal'u-sē),[key] —n., —pl.-cies. 1. a deceptive, misleading, or false notion, belief, etc.: That the world is flat was at one time a popular fallacy. 2. a misleading or unsound argument.
3. deceptive, misleading, or false nature; erroneousness.
4. Logic.any of various types of erroneous reasoning that render arguments logically unsound.