[fal-uh-see] 
| 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. |
| 5. | Obsolete. deception. |
| fal·la·cy (fāl'ə-sē) n. pl. fal·la·cies
[Alteration of Middle English fallace, from Old French, from Latin fallācia, deceit, from fallāx, fallāc-, deceitful, from fallere, to deceive.] |
A false or mistaken idea based on faulty knowledge or reasoning. For example, kings who have divorced their wives for failing to produce a son have held to the fallacy that a mother determines the sex of a child, when actually the father does. (See sex chromosomes.)