Definition from Wiktionary, the free dictionary
Alternative forms 
- A fallacy; a clearly defined error in reasoning used to support or refute an argument, excluding simple unintended mistakes.
- "You can't prove Santa doesn't exist, therefore Santa does exist", is a common logical fallacy.
- A formal fallacy.
- A valid syllogism contains three terms; with four it commits the logical fallacy known as quaternio terminorum.
clearly defined error in reasoning
See also 
- ^ Encyclopedia Britannica (1911), "Fallacy".