What is a second party check?

A "second party check" is a check where the payee is depositing or cashing the check. For example, Bob Smith writes a check to his brother John Smith. John Smith is the payee, (second party) who deposits the check into an account in his own name, or cashes the check.

If John Smith wants to endorse the check on the back to a different person, that other person would be a third party. The check would now be a "third party check." Banks are likely to not accept this type of check if the third party tried to cash it, because they cannot verify endorsements.