Accountancy Mark - Countries with bilateral revenue-sharing agreements used Accountancy Mark systems from 1843 to 1876. Marks on interpostal mail showed the amount of shared revenue between two or more postal entities. Accountancy Marks were shorthand notations for journal entry instructions to record amounts owing to, or amounts owing from other postal authorities. For prepaid mail, the calculation of shared revenues to be paid to the destination country was marked in red on outgoing mails, and for unpaid mail, the amount owed to the sender country for its share of unpaid mail was marked in black. Red was used to indicate a credit entry in the sending country account, and black was used to indicate a debit entry for an amount owed to the sending country. During the 33-year period, the system was changed from the recording of a simple manuscript or handstamp numeral marking, to PD, PP, and P abbreviation handstamps. The system was then modified to include Articles handstamps and Letter or Way Bills. Finally, a system of country-designated handstamps was used with specific amounts due.

Adhesive - A stamp that may be affixed to an article to prepay postal fees, in contrast to a design printed directly on an article, as with postal stationery. An adhesive can also refer to a registration label or other label added to a cover.

Advertising cover - An envelope used as a form of advertising. Businesses began using this form of advertising in the mid 1800s. The cachets, meant to communicate a certain prestige, could be as simple as a blind-embossed corner card, a fancy return address corner card, an illustration of buildings or products, or as fancy as an all-over advertisement. The advertising envelope is still with us today and is most often found on our bills and junk mail.

Aerogram - (Aerogramme) - Printed and gummed writing-sheet designed to be folded and sealed to form a lightweight air-mail letter. Usually made of thin paper and printed with the appropriate postal duty. No enclosures are permitted.

Aerogramme - Printed and gummed writing-sheet designed to be folded and sealed to form a lightweight air-mail letter. Usually made of thin paper and printed with the appropriate postal duty. No enclosures are permitted.

Airgraph - The airgraph was invented in the 1930's by the Eastman Kodak Company in conjunction with Imperial Airways and Pan-American Airways, as a means of reducing the weight and bulk of mail carried by air. The airgraph forms, upon which the letter was written, were photographed and then sent as negatives on rolls of microfilm. A General Post Office (GPO) poster of the time claimed that 1,600 letters on film weighed just 5oz, while 1,600 ordinary letters weighed 50 lbs. At their destination, the negatives were printed on photographic paper and delivered as airgraph letters through the normal Royal Engineers (Postal Section), also known as the Army Postal Services (APS), or Systems. In 1940, the British Minister of Transport, Lieutenant Colonel John Moore, put forward the idea that airgraphs be used to reduce both the bulk and weight of mail traveling between the Middle East Forces (MEF) and the UK. This eventually led to a service being instituted between England and Egypt in 1941, when 70,000 airgraphs were sent in the first batch and took three weeks to reach their destination.

Aerophilately - Area of stamp collecting which concentrates on mail carried by air.

Affranchts - Abbreviation for "Affranchissements". A pre-cancel marking found on French stamps used on Official Mail or on bulk postings of business mail.

Air letter - Special letter forms impressed with a stamp to facilitate the writing and handling of air mail letters.

Airmail - A type of mail transport using aircraft.

Air mail label - Member countries of the UPU adopted a standardized blue label for affixing to correspondence sent by air to facilitate easy recognition, also known as etiquettes.

Albino - A fortuitous omission of color from a stamp which results in a colorless impression of the design or overprint.

Ambulant - French word meaning 'moving' as used to denote a mobile or Traveling Post Office (TPO) in a cancellation.

AR - Acknowledgment of receipt - (equivalent terms include avis de réception (UPU term), advice of receipt (Canada), advice of delivery (UK )). The standard abbreviations are AR and AD. It is a postal service which returns to the sender of a letter (usually registered or insured) a form or card signed by the recipient. This serves as evidence that the letter was received.

Army Post Office (APO) - In times of war, the Civilian Postal Service delivered mail. A postmaster was assigned to each regiment and there was a post office on the battlefield for troops. It was not until World War I that the Army Post Offices were developed.

Arrival postmark - Impression placed on mail by receiving office to show name of office and arrival date. Usually applied on back of correspondence.

Auxiliary Marking - Postal marking applied to covers by handstamp, machine cancellation, a stick-on label, manuscript markings, or by mechanical or electronic methods, indicating that the covers were given special attention due to some special circumstance. A broad definition would include accompanying postal-service letters, ambulance covers in which damaged or misdirected mail has at times been delivered, and markings placed on covers by institutions other than the postal service, such as the military, prisons, hotels, etc. Auxiliary Markings are sometimes known as Instructional Markings or Supplementary Markings.