Henry's law, formulated by William Henry, states that the amount of a given gas dissolved in a liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
where k, the constant of proportionality, is termed Henry's constant.
This constant is, of course, dependent on the dimensions employed for the pressure and the concentration. In chemistry, atm and molarity are typical. A more elegant definition is to make it dimensionless making use of the ideal gas equation of state (if applicable).