Cohesion refers to the attractive forces (e.g., van der Waals, Coulombic) between two molecules of the same kind or two surfaces of similar material. For example, the shear strength of wet clayey soils involves both cohesive forces and friction. For wet clayey soils, the cohesion component of shear strength primarily results from the surface interactions between two clay particles (of similar composition and structure).
Cf., adhesion