fallacy
n. countablen. a false idea that many people believe is true, or a mistake in the way someone argues.
n. a failure in reasoning which renders an argument invalid; a deceptive or false notion. Often refers specifically to formal logical errors or widely held misconceptions.
It is a fallacy to think that money always brings happiness.
The politician's argument relied on a logical fallacy that assumed because two events happened together, one caused the other.
The sunk cost fallacy often compels investors to continue pouring resources into a failing venture simply because they have already committed so much to it.
From Middle English fallaci, fallace, fallas, from Old French fallace, from Latin fallācia (“deception, deceit”), from fallāx (“deceptive, deceitful”), from fallere (“to deceive”).
Commonly used with the definite article ('the fallacy') or as a complement after 'it is a...'.