According to WikiPedia…
In law, spoliation of evidence is the intentional or negligent withholding, hiding, alteration or destruction of evidence relevant to a legal proceeding. It is a criminal act in the United States under Federal and most State law. Spoliation has two consequences: first the act is criminal by statute and may result in fines and incarceration for the parties who engaged in the spoliation, secondly case law has established that proceedings which might have been altered by the spoliation may be interpreted under a spoliation inference. The spoliation inference is a negative evidentiary inference that a finder of fact can draw from a party’s destruction of a document or thing that is relevant to an ongoing or reasonably foreseeable civil or criminal proceeding: The finder of fact can review all evidence uncovered in as strong a light as possible against the spoliator and in favor of the opposing party.
The theory of the spoliation inference is that when a party destroys evidence, it may be reasonable to infer that the party had “consciousness of guilt” or other motivation to avoid the evidence. Therefore, the factfinder may conclude that the evidence would have been unfavorable to the spoliator. Some jurisdictions have recognized a spoliation tort action, which allows the victim of destruction of evidence to file a separate tort action against a spoliator.
Spoliation is often an issue in the context where a person claims he has been injured by a defective product which he then discarded or lost. In that circumstance, the defendant manufacturer or distributor may move to dismiss the case on the basis of spoliation (instead of just having to rely on the plaintiff’s usual burden of proof – plaintiff’s witnesses e.g. cannot make up for the lost product because of the spoliation exception).