the.com/vapor pressure

means The pressure exerted by a substance's vapor when it reaches equilibrium with its liquid (or solid) form at a given temperature — a measure of how readily molecules at the surface break free into gas.

from From Latin 'vapor' (steam, exhalation) joined to 'pressure,' from Latin 'pressura' (a pressing, squeezing). The term is a straightforward 18th–19th century scientific coinage, born as chemists and physicists began rigorously measuring the behavior of gases and liquids in sealed containers — no myth here, just thermodynamics getting a name for the tug-of-war between molecules wanting to fly off and the forces holding them in the liquid.