The official business definition of an invoice or purchase order is a document issued by a seller to a buyer, indicating the products, quantities, and agreed prices for products or services the seller has provided the buyer. An invoice indicates that the buyer must pay the seller, usually within a certain period of time. A purchase order is a document sent from a buyer to a seller, indicating types, quantities, and agreed prices for products or services the buyer agrees to purchase from the seller. It serves as an offer from the buyer to the seller, and it becomes a binding contract when the seller accepts the offer and confirms the purchase order. Both invoices and purchase orders are important documents in the business world, as they provide evidence of the sale and purchase of goods and services.