A postage stamp is a small piece of paper issued by a post office, postal administration, or other authorized vendors to customers who pay postage (the cost involved in moving, insuring, or registering mail). Then the stamp is affixed to the face or address-side of any item of mail—an envelope or other postal cover (e.g., packet, box, mailing cylinder)—which they wish to send. The item is then processed by the postal system, where a postmark or cancellation mark—in modern usage indicating date and point of origin of mailing—is applied to the stamp and its left and right sides to prevent its reuse. Next the item is delivered to its address.

Always featuring the name of the issuing nation (with the exception of the United Kingdom), a denomination of its value, and often an illustration of persons, events, institutions, or natural realities that symbolize the nation's traditions and values, every stamp is printed on a piece of usually rectangular, but sometimes triangular or otherwise shaped special custom-made paper whose back is either glazed with an adhesive gum or self-adhesive.

Because governments issue stamps of different denominations in unequal numbers and routinely discontinue some lines and introduce others, and because of their illustrations and association with the social and political realities of the time of their issue, they are often prized for their beauty and historical significance by stamp collectors, whose study of their history and of mailing systems is called philately. Because collectors often buy stamps from an issuing agency with no intention to use them for postage, the revenues from such purchases and payments of postage can make them a source of net profit to that agency. On 1 May 1840, Sir Rowland Hill invented the Penny Black, the first adhesive postage stamp issued in the United Kingdom, selecting a simple design that showed Queen Victoria's profile, the artist Henry Corbould draw her image, basing his work on a medal by William Wyon. Perkins, Bacon & Petch, a leading security printing firm printed the stamp. Within three years postage stamps were introduced in Switzerland and Brazil, a little later in the United States, and by 1860, they were in 90 countries around the world. The first postage stamps did not need to show the issuing country, so no country name was included on them. Thus the United Kingdom remains the only country in the world to omit its name on postage stamps; the monarch's image signifies the United Kingdom as the country of origin.

Throughout modern history numerous methods were used to indicate that postage had been paid on a mailed item, so several different men have received credit for inventing the postage stamp.

In April 1680, William Dockwra, an English searcher at the customs-house in London, and his partner Robert Murray established the London Penny Post. The LPP was a mail system that delivered letters and small parcels up to a pound in weight and £10 in value inside the city of London for the sum of one penny. Confirmation of paid postage was indicated by the use of a hand stamp to frank the mailed item. Though this "stamp" was applied to the letter or parcel itself, rather than to a separate piece of paper, it is considered by many historians to be the world's first postage stamp.

In 1835, the civil servant Lovrenc Košir from Ljubljana in Austria-Hungary (now Slovenia), suggested the use of "artificially affixed postal tax stamps" using "gepresste Papieroblate" ("pressed paper wafers"), but although civil bureaucrats considered the suggestion in detail, it was not adopted. The 'Papieroblate' were to produce stamps as paper decals so thin as to prevent their reuse.

In 1836, Robert Wallace, a Member of (British) Parliament, gave Sir Rowland Hill numerous books and documents about the postal service, which Hill described as a "half hundred weight of material". After a detailed study, on 4 January 1837 Hill submitted a pamphlet entitled Post Office Reform: Its Importance and Practicability to the Chancellor of the Exchequer, Thomas Spring Rice, which was marked "private and confidential", and not released to the general public. The Chancellor summoned Hill to a meeting at which he suggested improvements and changes to be presented in a supplement, which Hill duly produced and submitted on 28 January 1837. Hill wrote Post Office Reform while he served as secretary of the South Australian Colonization Commission. The colonization of South Australia roughly followed Edward Gibbon Wakefield's scheme of colonization which emphasized the uniform pricing of land, and Wakefield's ideas influenced Hill's argument for the uniform pricing of postal rates.

Summoned to give evidence before the Commission for Post Office Enquiry on 13 February 1837, Hill read from the letter he wrote to the Chancellor that included a statement saying that the notation of paid postage could be created... "by using a bit of paper just large enough to bear the stamp, and covered at the back with a glutinous wash..." This would eventually become the first unambiguous description of a modern adhesive postage stamp (though the term "postage stamp" originated at a later date). Shortly afterward, Hill's revision of the booklet, dated 22 February 1837, containing some 28,000 words, incorporating the supplement given to the Chancellor and statements he made to the commission, was published and made available to the general public. Hansard records that on 15 December 1837, Benjamin Hawes asked the Chancellor of the Exchequer "whether it was the intention of the Government to give effect to the recommendation of the Commissioners of the Post-office, contained in their ninth report relating to the reduction of the rates of postage, and the issuing of penny stamps?"