lighter than air

adjective Aeronautics.

1. (of an aircraft) weighing less than the air it displaces, hence obtaining lift from aerostatic buoyancy.
2. of or relating to lighter-than-air craft.

adjective (lighter-than-air when prenominal)

1. having a lower density than that of air
2. of or relating to an aircraft, such as a balloon or airship, that depends on buoyancy for support in the air