A money order is a piece of paper representing a sum of money which you can buy at a post office and send to someone as a way of sending them money by post.
[US]regional note: in BRIT, use postal order
money order in British English
noun
US and Canadian
a written order for the payment of a sum of money, to a named payee, obtainable and payable at a post office
Also called (in Britain and certain other countries): postal order
money order in American English
an order for the payment of a specified sum of money, as one issued for a fee at one post office, telegraph office, or bank and payable at another