lighter-than-air

The term is a modern English descriptive phrase from the field of aeronautics, coined in the late 18th or early 19th century following the development of hot-air and hydrogen balloons. It is built from core Germanic-derived English words: 'light' (from Old English lēoht, meaning "not heavy"), the comparative suffix '-er', the conjunction 'than' (from Old English þonne), and 'air' (from Old French air, ultimately from Latin āēr and Greek aēr). The phrase functions as a single conceptual unit to define the fundamental physical principle of buoyancy that allows certain craft to float in the atmosphere, contrasting with 'heavier-than-air' flight which requires aerodynamic lift.

This is a fixed comparative adjectival phrase. It is not a single, segmentable word but a compound descriptor formed from the comparative adjective 'lighter', the conjunction 'than', and the noun 'air'.