noun
self-addressed stamped envelope; SASE
An envelope provided for the recipient to send a reply, typically with the sender's address and postage already affixed.
申込書には返信用の封筒を同封してください。
Please enclose a self-addressed stamped envelope with the application form.
Compound of 返信 (reply) + 用 (for the purpose of) + の (possessive particle) + 封筒 (envelope).