The clear, yellowish fluid portion of blood, lymph, or intramuscular fluid in which cells are suspended. It differs from serum in that it contains fibrin and other soluble clotting elements. Blood plasma. Medicine Cell-free, sterilized blood plasma, used in transfusions. Protoplasm or cytoplasm. The fluid portion of milk from which the curd has been separated by coagulation; whey. Physics An electrically neutral, highly ionized gas composed of ions, electrons, and neutral particles. It is a phase of matter distinct from solids, liquids, and normal gases.