At constant temperature and pressure, the ratio of the partial pressure of a gas above a liquid to its solubility in the liquid and therefore a measure of its partition between the gas phase and the solute phase.

Note 1: The solubility may be expressed in any convenient units, such as amount fraction, molality or amount (substance) concentration. The exact definition used should always be given.

Note 2: Rigorously, the Henry’s law constant is the limiting value at zero partial pressure.