To deplane is to get off of an airplane. It is a verb that is conjugated through all the tenses. It was coined in the 1920s and is an odd usage of the prefix de-. While it is used in some words to mean removal, such as dehumidify, in deplane it is not the plane that is being removed, but the person from the aircraft. Deplane is more commonly used inside the United States.
A synonym for deplane is disembark, which means to get off of an aircraft or ship. But disembark also carries the meanings of a ship or plane leaving the shore or land.