It is defined as the relative strength of the surface of the stationary phase in adsorption chromatography.  In the case of silica gel, the more silanol groups are available on the surface, the more active it is.  One can modify activity by adding water or other polar solvent that will bond to active sites, reducing activity.  

Cogent TYPE-C™ HPLC columns have very low activity due to the virtual absence of silanol functional groups on the surface.