In law, void means of no legal effect. The latin phrase, void ab initio means, to be treated as invalid from the outset. An action, document or transaction which is void is of no legal effect whatseover; an absolute nullity.

For example, in many jurisdictions where a person signs a contract under duress, that contract is treated as being void ab initio. A contract is a promise or an agreement that is enforced or recognized by the law. ... Duress in the context of contract law is a common law defence, and if you are successful in proving that the contract is vitiated by duress, you can rescind the contract, since it is then voidable. ...

In practical terms, void is usually used in contradistinction to voidable and unenforceable, the principle difference being that an action which is voidable remains valid until it is avoided. The significance of this usually lies in the possibility of third party rights being acquired. For example, in Cundy v Lindsay (1878) 3 App Cas 459 a fraudster induced Messrs Lindsay & Co to sell to him a quantity of handkerchiefs. The fraudster then sold the handkerchiefs on to an innocent third party, Mr Cundy. The fraudster was convicted, and the money was never recovered from him. Lindsay sued claiming ownership of the handkerchiefs. If the contract of sale was held to be voidable for fraud, the fraudster could pass good title Mr Cundy, and Lindsay & Co would only have recourse against the insolvent fraudster. However, if (as was in fact held) the contract of sale was void ab initio, then title did not pass, and Lindsay could claim back the handkerchiefs as their property, and Mr Cundy was left with only a claim against the insolvent fraudster.

However, the right to avoid a voidable transaction can be lost (usually lost by delay). These are sometimes referred to as "bars to rescission". Such considerations do not apply to matters which are void ab initio.

Unenforceable transactions essentially remain valid so long as they are performed, but the court will not assist a party in the even of non-performance.