The official business definition of an invoice vs. a purchase order is that an invoice is a document that a seller sends to a buyer to request payment for goods or services. It typically includes the amount due, the date payment is due, and the terms of payment. A purchase order, on the other hand, is a document sent from a buyer to a seller to request goods or services. It typically includes the item description, quantity, price, delivery date, and payment terms. Both documents are important for businesses to have in order to ensure that goods and services are delivered and paid for in a timely manner. An invoice is used to request payment for goods or services that have already been delivered, while a purchase order is used to initiate the purchase of goods or services.