当前国际贸易大多数以单证为媒介，并且通过单证的交接和传递来实现和信用证（Letter of Credit）是国际货物买卖中最重要的一种支付方式，指开证银行根据买方的指示和要求开立的、以卖方为受益人，保证在卖方履行信用证规定各项条件、交付规定的单据时支付一定金额的书面承诺。为了更好的理解该词，以下是该词的英文释义：A letter of credit, or “credit letter” is a letter from a bank guaranteeing that a buyer’s payment to a seller will be received on time and for the correct amount. In the event that the buyer is unable to make a payment on the purchase, the bank will be required to cover the full or remaining amount of the purchase. It may be offered as a facility.

如：

A letter of credit by its nature is a separate transaction from the sale or other contract on which it may be based. Banks are in no way concerned with or bound by such contract, even if any reference whatsoever to it is included in the credit. Consequently, the undertaking of a bank to honour, to negotiate or to fulfill any other obligation under the credit is not subject to claims or defenses by the applicant resulting from its relationships with the issuing bank or the beneficiary.

就性质而言，信用证与可能作为其依据的销售合同或其它合同，是相互独立的交易。即使信用证中提及该合同，银行亦与该合同完全无关，且不受其约束。因此，一家银行作出兑付、议付或履行信用证项下其它义务的承诺，并不受申请人与开证行之间或与受益人之间已有关系下产生的索偿或抗辩的制约。