The tendency of the rocks solid material to imbibe one fluid (known as "wetting phase") in the presence of another fluid (known as "non-wetting phase").
See also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petrophysics
[ Permeability ] [Relative permeability @model ]
Reference
[1] Fundamentals of Wettability - Oilfield Review - 2007