Undue influence occurs when one party to a contract takes advantage of their position of power over another party, persuading them to enter into a contract that they might not have agreed to otherwise. This influence can arise from a pre-existing relationship, such as parent-child or advisor-client.

In situations where undue influence is established, the affected party may seek to have the contract set aside by demonstrating that their consent was not freely given. The burden of proof may shift to the party exerting influence to show that the other party had independent advice and understood the contract fully.

Undue influence may be classified into two types: actual undue influence, where the influence is directly proven, and presumed undue influence, which is inferred from the relationship between the parties. In contracts affected by undue influence, the law aims to restore fairness and protect vulnerable parties.