A logical fallacy can be any one of a number of formal or informal mistakes in a deductive proof.
Note that an argument can be fallacious but still correct. For instance:
- All cats are mammals.
- All cats are dogs.
- Therefore all dogs are mammals.
The conclusion is accurate even though the premises and deductive method are incorrect.
Likewise, an argument can be logically sound but still wrong.
- All bugs are insects.
- All spiders are bugs.
- Therefore, all spiders are insects.
Some of the premises are factually incorrect. However, the conclusion based is an accurate deduction based on those premises.