fal·la·cy 
(fălə-sē)
(fălə-sē)n. pl. fal·la·cies
1. A false notion.
2. A statement or an argument based on a false or invalid inference.
3. Incorrectness of reasoning or belief; erroneousness.
4. The quality of being deceptive.
[Alteration of Middle English fallace, from Old French, from Latin fallācia, deceit, from fallāx, fallāc-, deceitful, from fallere, to deceive.]