Definition
Flight achieved by an aircraft whose total weight is less than the weight of the air it displaces, allowing it to rise and remain aloft through buoyancy rather than aerodynamic lift. Examples include hot air balloons, gas balloons, and airships (dirigibles).
Plain English
Flying in a craft that floats in the air, the way a boat floats on water, because the craft as a whole weighs less than the air around it.
Context Anchor
Seen in aviation history when comparing balloons and airships with heavier-than-air airplanes.
Derivation
The phrase comes straight from physics: the aircraft is literally lighter than an equal volume of surrounding air. Heating air (as in a hot air balloon) or filling an envelope with a light gas like helium or hydrogen makes the craft less dense than the air it sits in, so it rises.
Why Pilots Care
It explains a whole branch of aviation that does not rely on wings or engines for lift. Knowing the distinction helps a pilot understand why balloons and airships behave very differently from airplanes in the same weather conditions.
Analogy
Like a cork in water. The cork floats not because anything pushes it up actively, but because it is lighter than the water around it. A balloon floats in air for the same reason.
Intuition Check
Lighter-than-air does not mean the aircraft has no weight. It means the aircraft is light enough overall that the surrounding air can support it upward.
Example Sentence 1
The history of lighter-than-air flight began in 1783 with the Montgolfier brothers' hot air balloon.
Example Sentence 2
Modern blimps rely on lighter-than-air flight for extended low-speed observation over crowds.