发票，是指经济活动中，由出售方向购买方签发的文本，内容包括向购买者提供产品或服务的名称、品质、协议价格。除了预付款以外，发票必须具备的要素是根据议定条件由购买方向出售方付款，必须包含日期和数量，是会计账务的重要凭证。发票也通常称为付款单。

在中国大陆，付款单、收款收据在民间有时也被统称为发票。当局的会计制度规定有效的购买产品或服务的发票称为税务发票。政府部门收费、征款的凭证各个时期和不同收费征款项目称呼不一样，但多被统称为行政事业收费收款收据。

为内部审计及核数，每一张发票必须有独一无二的流水账号码，防止发票重复或跳号。

See also Voucher: an invoice is within the European union 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, 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 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 in which 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.