Meaning of fallacy

fal•la•cy

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. any of various types of erroneous reasoning that render arguments logically unsound.
5. deception.