Pheresis: Procedure in which the blood is filtered, separated, and a portion retained, with the remainder being returned to the individual.
There are various types of pheresis. In leukapheresis, the leukocytes (white blood cells) are removed. In plateletpheresis, the thrombocytes (blood platelets) are removed. And in plasmapheresis, the liquid part of the blood (the plasma) is removed.
From the Greek "aphairesis" meaning withdrawal.