Cash is a form of money that offers final settlement of any transaction or debt, without credit or counterparty risk and without the need for any third-party involvement. Physical cash transactions are naturally peer-to-peer since they take place directly between two participants and no one else needs to know or otherwise be involved.
Final settlement refers to the assurance the receiver can have that they have actually received the money. In a physical cash transaction this is achieved by money changing hands. Counterparty risk exists in credit transactions and refers to the assurance that the counterparty is ‘good for it’ – i.e., has the cash to pay. In a physical cash transaction this is avoided by money actually changing hands.