Final answer:
A contract based on illegal consideration is one where the subject matter or purpose of the contract has an illegal nature or is against the law. It is void and unenforceable.
Step-by-step explanation:
When a contract is based on illegal consideration, it means that the subject matter or the purpose of the contract has an illegal nature or is against the law. In such cases, the contract is considered void and unenforceable. Illegal consideration can vary depending on the jurisdiction, but examples include contracts related to illegal activities like drug trafficking, prostitution, or fraud.