From Latin 'fluidus' (flowing), from 'fluere' (to flow) — both noun (a flowing substance) and adjective (not fixed).
A substance that has no fixed shape and yields easily to external pressure; a gas or (especially) a liquid; not fixed or stable; able to flow.
From Latin 'fluidus' (flowing, fluid, moist, unstable), an adjective from 'fluere' (to flow, to stream, to run), from PIE *bhleu- (to swell, to overflow, to flow). The Latin verb 'fluere' is the source of one of the richest semantic clusters in English: 'fluent' (flowing speech), 'flux' (continuous flow or change), 'fluctuate' (to move in waves), 'influence' (originally an astral flowing-in that shaped earthly affairs), 'influenza' (named for astrological influence), 'effluent' (flowing out), 'affluent' (flowing toward, wealthy — riches flow in), 'confluence' (flowing together), 'reflux' (flowing back), and 'superfluous' (flowing over and above what is needed). The PIE root *bhleu- also underlies Germanic
In physics, both liquids and gases are classified as 'fluids' because both flow and deform continuously under applied shear stress. This means that technically, the air you breathe is a fluid. The discipline of 'fluid dynamics' studies both — from ocean currents to wing aerodynamics — all under the Latin