A voucher for credit for goods or services. 
A document issued that reduces the amount of a receivable or increases the amount of a payable. These are used to record returned goods, defective goods or services, miscellaneous charges or credits, and error corrections.
A credit memo, also called a memorandum, is a document issued by a seller that reduces the amount owed by a client from a previous invoice. This means that whatever the client owes to the seller will decrease after this memo is issued.