UV pinning is the process of applying a dose of low intensity ultraviolet (UV) light to a UV curable ink (UV ink).  The lights wavelengths must be correctly matched to the inks photochemical properties.  As a result, the ink droplets move to a higher viscosity state, but stop short of full cure.  This is also referred to as the "gelling" of the ink.
UV pinning is typically used in UV ink jet applications (e.g. the printing of labels, the printing of electronics, and the fabrication of 3-D microstructures).