Final answer:
Joe and Josh's agreement to exchange widgets for payment upon delivery constitutes a contract, which is a mutual agreement that creates specific obligations enforceable by law.
Step-by-step explanation:
The scenario described involves Joe agreeing to deliver 10 boxes of widgets in exchange for Josh's promise to pay $50 per box. Once Joe delivers the widgets and Josh pays for them, they have completed a transaction. This scenario is an instance of a contract, which is a mutual agreement between two or more parties that creates enforceable obligations. A contract involves specific rights and obligations between the parties involved. In this case, Joe's obligation was to deliver the widgets, and Josh's obligation was to pay for them upon delivery. Their fulfillment of these promises constitutes the successful execution of their agreement.