reply postcard; international reply coupon
A postcard pre-printed or designed for the recipient to send a reply, often with postage already paid. In international contexts, it can refer to an international reply coupon (IRC).
Please enclose a reply postcard with the application form.
International reply coupons are used when requesting a reply from abroad.
A round-trip postcard consisting of two attached cards: one for the outgoing message and one for the reply. 返信用葉書 is specifically the reply half or a standalone reply card.
Compound of 返信 (reply), 用 (for the purpose of), and 葉書 (postcard).