Acknowledgement of receipt stamp - forms used as prepayment on notices attesting to the delivery and receipt of mail

Adhesive - a substance applied to the back of most stamps to facilitate attaching them to the mailing surface. Adhensives are both water-activated and pressure-sensitive (self-adhesive). Adhesives are described according to color, texture, pattern, or method of application. Gum is one such substance.

Adhesive revenue stamp - 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.

Adversity cover - letter writing material, including envelopes, used when paper supplies were in short supply. For example, during the Civil War, the Union blockade proved critical in restricting goods from entering and leaving the Confederacy. Southerners faced increasing shortages of supplies, including paper and envelopes. Writers began to use whatever was handy as letter writing paper and envelopes. These items are known by philatelists as "adversity covers." Letters and envelopes were fashioned from the backs of ledger sheets, printed circulars, blank pages in books, maps and even wallpaper torn from walls. Some writers re-used envelopes by turning them inside out. Any blank or partially blank piece of paper could be pressed into service as an envelope.
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 product, 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.

Airmail envelope - an envelope transported by aircraft

Airmail pilots - a pilot who flew the mail. The profession of airmail pilot was, in 1918 America, a horrendously dangerous one. A surviving pilot recalled that the group was "considered pretty much a suicide club." Aviation was still in its infancy. Few planes offered protection and crashes were common. The service had to prove itself from the outset. Flight schedules were controlled by the Post Office Department and officials strove to keep schedules tight regardless of weather conditions. Pilots flew without parachutes over land that had few, if any, emergency landing fields. There were no lights on the ground or in the plane to assist with night flights; no wireless weather reports; and no wing de-icers or radio guides. The life expectancy of the first mail pilots was as short as 900 flying hours. Thirty-one of the forty pilots hired by the Post Office Department between 1919 and 1926 were killed while flying the mail. Most of those pilots died in the early years of the service. In 1919, one pilot died for every 115,325 miles flown. By 1926, the number had dropped to one pilot death for every 2,583,056 miles flown.

Airmail service - a type of mail transport using aircraft. The United States government instituted regular scheduled airmail service between New York, Philadelphia, and Washington on May 15, 1918. Coast-to-coast service began in 1920. Airmail has been carried under contract since 1926, with fast, efficient service resulting from extensive progress in the commercial airline industry.
Airmail stamp - a stamp prepaying postal rates for air transport. Italy issued the first airmail stamp in 1917.

Airway letter stamp - a local stamp issued by British European Airways (later British Airways) and also Cambrian Airways for transporting letters between airports

Arc roulette - a separation in which curved cuts appear as a semi-circle

Army Post Office (APO) - During the Civil 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. When the Spanish-American War began, with soldiers fighting outside the United States, the Civilian Postal Service followed them. It wasn't until World War I that the Army Post Offices were developed. These were still operated by the Civilian Postal Service, but with assistance from the troops themselves. By the end of WWI there were a hundred sixty-nine Army Post Offices located in France. The first APO (Army Post Office) was opened on July 10, 1917. From WWI through current conflicts, military post offices have helped move mail to military personnel. In 1917, when the first APOs were established, the civilian postal service worked the mail with the assistance of U.S. troops. By the end of the war, there were a hundred sixty-nine APOs in France moving mail to and from American troops stationed in Europe. The military's mail was placed under their control in 1940, when Congress established the Army Postal Service. This new organization continued to work with the U.S. Post Office Department to keep mail moving between the troops and their loved ones back home.

Arrow - marking in a margin, as a guide for cutting sheets into smaller units for perforations. After 1894, arrows were used at both ends of guide lines of U. S. stamps printed from flat plates.
Authorized delivery stamp - denotes the tax on mail permitted to be delivered by private services
Authorized non-profit organization stamp - a stamp issued by U.S. Post Office to prepay special concessionary postage rates on correspondence of charities and other institutions. See also 'bulk rate stamps'.

Autogiro mail service - a service using experimental aircraft that carried mail over short, but congested routes. The postal service placed an autogiro aircraft (a rotary-winged aircraft with a freely rotating main rotor) into use flying mail between Camden, New Jersey, and Philadelphia, Pennsylvania. A series of 'rehearsal' flights were made before the official flight on July 6, 1939. The New York Times reported on July 3, 1939, that these rehearsal flights took only 6.5 minutes, easily besting the time needed by mail trucks covering the same forty to forty-five mile route. On the first day of service, 52,128 first-flight covers were cancelled and carried on the flight for philatelists, many of whom paid double the 6-cent stamp price to cover postage for a round trip. The postal service made over $3,000 in revenue from the flights. The autogiros were put to use flying airmail in Chicago, New Orleans, Los Angeles, and Washington, D.C., into the 1940s. Helicopter airmail service eclipsed autogiro service.

Auxiliary mark - secondary or supplementary marking that requires the mailer or addressee to take further action on a piece of mail; for example, a mark which notifies the recipient of postage due