A fallacy is an error in reasoning or logic that leads to a false conclusion. It's a misguided belief, mistaken idea, or deceptive argument that appears to be logical but actually contains a mistake. Fallacies can occur in various forms such as ad hominem attacks, straw man arguments, and slippery slope fallacies among others. They are often used intentionally by individuals to manipulate or deceive others, or unintentionally due to lack of knowledge or understanding.