In general, any error or misconception.
In contract law, a situation where the parties did not mean the same thing when they agreed to a term or provision. Also, when at least one contracting party held a belief that was factually or legally false. As a result, the contract may be subject to modification or rescission.
In criminal law, a defense that claims either a misconception of the law or some fact. The defense of mistaking the law almost never succeeds. Mistakes of fact can work as a defense more often, especially if they are reasonable.