What is "international reply coupon"

n. A prepaid international postage voucher.

An international reply coupon (IRC) is a coupon that can be exchanged for one or more postage stamps representing the minimum postage for an unregistered priority airmail letter of up to twenty grams sent to another Universal Postal Union (UPU) member country. IRCs are accepted by all UPU member countries.

UPU member postal services are obliged to exchange an IRC for postage, but are not obliged to sell them.

The purpose of the IRC is to allow a person to send someone in another country a letter, along with the cost of postage for a reply. If the addressee is within the same country, there is no need for an IRC because a self-addressed stamped envelope (SASE) or return postcard will suffice; but if the addressee is in another country an IRC removes the necessity of acquiring foreign postage or sending appropriate currency.