A postcard or post card is a rectangular piece of thick paper or thin cardboard intended for writing and mailing without an envelope and at a lower rate than a letter. Stamp collectors distinguish between postcards (which require a stamp) and postal cards (which have the postage pre-printed on them). While a postcard is usually printed by a private company, individual or organization, a postal card is issued by the relevant postal authority. The United States Postal Service defines a postcard as: rectangular, at least 3-½ inches high x 5 inches long x .007 inch thick and no more than 4-¼ inches high x 6 inches long x .016 inches thick;1 (in metric; 12.7 cm x 8.9 cm) however, some postcards have deviated from this (for example, shaped postcards).

The study and collecting of postcards is termed deltiology.

Brief history of postcards in the United States

John P. Charlton of Philadelphia patented the postcard in 1861, selling the rights to H. L. Lipman, whose postcards, complete with a decorated border, were labeled "Lipman's postal card." Nine years later European countries were also producing postcards.

The United States Postal Service began issuing pre-stamped postal cards in 1873. The postal cards came about because the public was looking for an easier way to send quick notes. The USPS was the only establishment allowed to print postcards, and it held its monopoly until May 19, 1898, when Congress passed the Private Mailing Card Act which allowed private publishers and printers to produce postcards.

The first postcard in the United States was created in 1893 to advertise the World's Columbian Exposition in Chicago. Shortly thereafter the United States government, via the United States Postal Service, allowed printers to publish a 1-cent postcard (the "Penny Postcard"). A correspondent's writing was allowed only on the front side of these cards.

1901 brought cards with the word "Post Card" printed on the reverse (the side without the picture). Written messages were still restricted to the front side, with the entire back dedicated to the address. This "undivided back" is what gives this postcard era its name.