An invoice is a commercial document that outlines the details of a transaction between a seller and a buyer. It serves as a request for payment from the seller to the buyer. Invoices typically include information such as the description and quantity of services provided, the agreed-upon price, any applicable taxes or discounts, and the terms of payment. They also include the names and contact information of both the seller and the buyer, as well as a unique invoice number and the date of issuance.