A property of gases, which states:
The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume.
In other words, the pressure of a gas mixture (such as air) is the sum of the pressures of all of the individual gases (which are termed the partial pressures of those gases).
This is relevant to scuba diving because it means that as air is pressurized, the partial pressure of each component gas increases. The combination of this with Henry's Law explains nitrogen narcosis and decompression sickness.