Lighter than air

weighs less than air, has very little weight, is very light

Some gases are buoyant in air because they have a density that is less than the density of air (about 1.2 kg/m3, 1.2 g/L). Lighter than air gases are used to fill craft called aerostats which include free balloons, moored balloons, and airship to make the whole aircraft, on average, lighter than air. (Heavier than air aircraft include aeroplanes, gliders and helicopters.)
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Adjective
1. relating to a balloon or other aircraft that flies because it weighs less than the air it displaces
(similar) light
(classification) aircraft

lighter-than-air /'laitəðən'eə/
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