fallacy

Concise Oxford English Dictionary © 2008 Oxford University Press:
fallacy /ˈfaləsi/

▶noun (pl. fallacies)

• 1 a mistaken belief.

• 2 Logic a failure in reasoning which renders an argument invalid.

– derivatives
fallacious adjective,
fallaciously adverb,
fallaciousness /fəˈleɪʃəsnɪs/ noun.

– origin C15 (in the sense ‘deception, guile’): from L. fallacia, from fallax, fallac- ‘deceiving’.