n. [ L., akin to Gr. &unr_;&unr_;&unr_;, Skr. sāra curd. ] (Physiol.) (a) The watery portion of certain animal fluids, as blood, milk, etc. (b) A thin watery fluid, containing more or less albumin, secreted by the serous membranes of the body, such as the pericardium and peritoneum. [ 1913 Webster ]

Blood serum, the pale yellowish fluid which exudes from the clot formed in the coagulation of the blood; the liquid portion of the blood, after removal of the blood corpuscles and the fibrin. --
Muscle serum, the thin watery fluid which separates from the muscles after coagulation of the muscle plasma; the watery portion of the plasma. See Muscle plasma, under Plasma. --
Serum albumin (Physiol. Chem.), an albuminous body, closely related to egg albumin, present in nearly all serous fluids; esp., the albumin of blood serum. --
Serum globulin (Physiol. Chem.), paraglobulin. --
Serum of milk (Physiol. Chem.), the whey, or fluid portion of milk, remaining after removal of the casein and fat. [ 1913 Webster ]

n. (Med.) The treatment of disease by the injection of blood serum from immune animals. [ Webster 1913 Suppl. ]