A QSL card is a written confirmation of either a two-way radio communication between two amateur radio stations or a one-way reception of a signal from an AM radio, FM radio, television or shortwave broadcasting station. It can also confirm the reception of a two-way radiocommunication by a third party listener.
QSL card derived its name from the Q code “QSL”. A Q code message can stand for a statement or a question (when the code is followed by a question mark). In this case, ‘QSL?’ (note the question mark) means “Do you confirm receipt of my transmission?” while ‘QSL’ (without a question mark) means “I confirm receipt of your transmission.”
A typical QSL card is the same size and made from the same material as a typical postcard, and most are sent through the mail as such.