1. n. 
A dimensionless term devised to adapt the Darcy equation to multiphase flow conditions. Relative permeability is the ratio of effective permeability of a particular fluid at a particular saturation to absolute permeability of that fluid at total saturation. If a single fluid is present in a rock, its relative permeability is 1.0. Calculation of relative permeability allows comparison of the different abilities of fluids to flow in the presence of each other, since the presence of more than one fluid generally inhibits flow.
2. n. 
A measurement of the ability of two or more fluid phases to pass through a formation matrix. The relative permeability reflects the capability of a specific formation to produce a combination of oil, water or gas more accurately than the absolute permeability of a formation sample, which is measured with a single-phase fluid, usually water.