- serum [sir′əm]n.pl. serums or sera [sir′ə] [L, whey < IE * serom, fluid < base * ser-, to flow > Sans sara-, fluid, Gr hormē, an attack]1.a) a clear, watery animal fluid, as serous fluidb) the clear, yellowish fluid of the blood which separates from a blood clot after coagulation and shrinkage: in full blood serum2. blood serum containing agents of immunity, taken from an animal made immune to a specific disease by inoculation: it is used as an antitoxin and for diagnosis3. the whey of milk4. the thin, watery part of a plant fluid
English World dictionary. V. Neufeldt. 2014.