Footnote

a This type is known as a zeppelin, or a rigid airship, since it has a rigid frame, which maintains the shape of the vehicle. The nonrigid airship​—sometimes called a blimp—​has no frame but simply consists of a balloonlike bag kept in shape solely by the pressure of the gas inside. The third type is the semirigid airship, similar to the nonrigid but with the addition of a fixed keel under the envelope of gas. The common feature of all airships that distinguishes them from balloons is the motor, which enables airships to be steered.