A property characterising agility of the  fluid -phase under pressure gradient with account of reservoir permeability and dynamic fluid viscosity:

\frac{k_\alpha}{\mu_\alpha} = k_{air} \cdot \frac{k_{r \alpha}}{\mu_\alpha}

where 

absolute permeability to air


relative phase mobility


In most practical case of a 3-phase fluid model this will be:

oil mobility

gas mobility


water mobility