Partial Pressure
The pressure that one component of a gas mixture would exert if it alone occupied the whole volume.
Formula / equation
p_i = x_i × p_total
Unit
Pa
In depth
Partial pressure equals mole fraction times total pressure, so it measures how much of the mixture a component represents. Gases dissolve, diffuse and react according to their partial pressures rather than their absolute amounts, which is why equilibrium constants for gas reactions use K_p.
Examples in the real world
Oxygen's 21 kPa partial pressure at sea level drops to about 7 kPa at the summit of Everest.