serum

noun pl. serums or sera

1. 1. a clear, watery animal fluid, as serous fluid
   2. the clear, yellowish fluid of the blood which separates from a blood clot after coagulation and shrinkage
      in full blood serum
2. 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 diagnosis
3. the whey of milk
4. the thin, watery part of a plant fluid