An invoice, bill or tab is a commercial document issued by a seller to a buyer relating to a sale transaction and indicating the products, quantities, and agreed-upon prices for products or services the seller had provided the buyer.

Payment terms are usually stated on the invoice. These may specify that the buyer has a maximum number of days to pay and is sometimes offered a discount if paid before the due date. The buyer could have already paid for the products or services listed on the invoice. To avoid confusion and consequent unnecessary communications from buyer to seller, some sellers clearly state in large and capital letters on an invoice whether it has already been paid.

From a seller's point of view, an invoice is a sales invoice. From a buyer's point of view, an invoice is a purchase invoice. The document indicates the buyer and seller, but the term invoice indicates money is owed or owing.

Within the European Union, an invoice is primarily legally defined by the EU VAT directive as an accounting voucher (to verify tax and VAT reporting) and secondly as a Civil law (common law) document.

An invoice should not be mixed up with Proforma Invoice that indicates commitment, intention, or prepayment request.