self-addressed stamped envelope; SASE
A reply envelope with the sender's own address and postage already affixed, commonly enclosed with business correspondence or applications.
Please enclose a self-addressed stamped envelope with the application form.
封筒 is a general envelope; 返信用封筒 specifically means a reply envelope with postage and address already provided.
Compound of 返信 (reply) + 用 (for the purpose of) + 封筒 (envelope).