A stamp is a small piece of paper that can be purchased and displayed on the mail as proof of postage. Usually, stamps are printed on special custom-made paper with the country name and face value printed on the front and adhesive or self-adhesive on the back. Stamps are purchased from the Postal Administration (post office) or other authorized suppliers to cover the costs involved in the transportation of mail and other business necessities such as insurance and registration. They are sometimes a source of net profit for issuers, especially when sold to collectors who do not actually use them to collect postage.

Stamps are usually rectangular, but occasionally use triangles or other shapes. Stamps are attached to envelopes or other postal covers (e.g. small bags, boxes, mailboxes) that customers want to send. The item is then processed by the postal system, where the postmark (sometimes referred to as unmarking) is usually applied to stamps and covers in an overlapping manner. This process marks the seal to prevent its reuse. In modern usage, a postmark usually indicates the date and starting point of a message. The mailed message will then be sent to the address that the customer has applied to the envelope or package.

Since the 1840s, stamps have facilitated the delivery of mail. Prior to this, ink, usually made of wood or cork, and hand stamps (hence the term "stamp") were commonly used to mail and confirm that postage had been paid. In 1840, the first self-adhesive stamp was issued in England, usually called "Penny Black". The invention of stamps was part of an attempt to reform and improve the postal system in the United Kingdom of Great Britain and Ireland, which became chaotic and corrupt in the early 19th century. The inventor of stamps has many different opinions.

Before the introduction of stamps, mail in the UK was paid by the addressee, a system related to an unsolvable problem: when the addressee is unable or unwilling to pay, the postal service cannot recover the cost of mailing. Delivered items, and the sender has no incentive to limit the number, size, or weight of items sent, whether or not payment is final. Stamps solve this problem in a simple and elegant way, and provide additional benefits for the introduction of aesthetics. The first stamp was accompanied by a package. Later related inventions included Postal Stationery, such as prepaid envelopes, postcards, stationery, aeronautical charts, postage meters, and recently dedicated cartons and envelopes for priority or express delivery that were provided free of charge by the U.S. Postal service to customers.

Stamps facilitate postmen and postal officials, more effectively recover the cost of postal services, and eventually establish a better and faster postal system. By providing convenient stamps, their use greatly increased the number of posts in the 19th and 20th centuries. In this era, stamps are the most popular way of payment by post. By the end of the 20th century, however, the use of metered postage and large volumes of mail by businesses had quickly overshadowed them.

With the widespread use of stamps with engraved images, historians and collectors began to notice this. The study and use of stamps is called philately. Stamp collecting can be a hobby or a form of historical research and reference, because the stamps issued by the government and their mailing system have always been closely related to national history.