A postcard or post card is a rectangular piece of thick paper or thin cardboard intended for writing and mailing without an envelope.
In some places, it is possible to send them for a lower fee than for 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 31⁄2 inches (88.9 mm) high × 5 inches (127 mm) long × 0.007 inches (0.178 mm) thick and no more than 41⁄4 inches (108 mm) high × 6 inches (152.4 mm) long × 0.016 inches (0.406 mm) thick. However, some postcards have deviated from this (for example, shaped postcards).
The study and collecting of postcards is termed deltiology.