The fine crystals that fall out of marijuanna when it is left in a container and shaken. You can buy crystal catchers which advertise mesh screens for catching crystals which can be turned into pollen by rubbing the crystals on the mesh screen. Pollen is usually collected in a catcher and pressed or sprinkled over a joint. It is smooth to smoke and has a more concentrated THC amount.
My grinder has four compartments, one of which is a pollen catcher.