money order

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An order for the payment of a specified amount of money, usually issued and payable at a bank or post office.

from The Century Dictionary.

• noun An order, payable at sight, granted, upon payment of the sum and a small commission, by one post-office, and payable at another.
• noun A money-order post-office.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A type of cheque, usually purchased from a postal service to allow safe sending of money through the post, possibly to be redeemed in another country.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a written order for the payment of a sum to a named individual; obtainable and payable at a post office