A refutation proves that something is false. Refutations pop up often in law debates and philosophical arguments.
While a validation tells you something is true, a refutation does the opposite: it says or proves that something is untrue, refuting the claim. In court, a witness might offer a refutation of a suspect's alibi to show he's lying. If someone calls you a liar, you probably should give them a refutation — make the case that you're a person who tells the truth.