A mistake in contract law refers to an incorrect belief held by one or both parties about a material fact relevant to the agreement. It can lead to a contract being voidable if it significantly affects the parties' understanding or intentions. Mistakes can be classified as mutual, where both parties share the same erroneous belief, or unilateral, where only one party is mistaken. Understanding how mistakes interact with contract terms is crucial when considering the Parol Evidence Rule, which governs the admissibility of external evidence in interpreting contracts.