Acknowledgment of receipt (equivalent terms include avis de réception (UPU term), Aviso de Recibo, advice of receipt, advice of delivery (UK and much of the Commonwealth), return receipt requested/required/wanted/demanded (US), Rückschein (Germany), ricevuta di ritorno (Italy), zwrotne potwierdzenie odbioru (Poland), and many others; the standard abbreviations are AR and AD) is a postal service which returns to the sender of a letter (usually registered; in the case of a parcel, it may be insured) a form or card signed by the recipient. This is evidence that the letter was received, and these forms (or cards) are frequently seen with legal endorsements (exhibit A, for example).
While obscure in some countries (such as Canada, Great Britain and Australia), AR has always been very popular in others (such as India, France and its overseas departments and territories, and especially the US). There is a great deal of postal history material associated to AR: postmarks, AR forms & cards, AR covers, AR covering envelopes for returning AR forms (in use until the early to middle 1920s), rates, and practices. For (a lot) more information, see the book, AR---avis de réception by David Handelman, published by the Postal History Society of Canada (2002) Ottawa.
According to op.cit., six postal entities have issued stamps intended to pay the AR fee.