Definition of: serum

(sir′əm) noun plural se·rums or se·ra (sir′ə)
1. The more fluid constituent of blood, lymph, milk, and similar animal liquids.
2. The serum of the blood of an animal which has been subjected to the process of immunization; any antitoxic blood serum or lymph.
3. Whey; serum of milk.
4. Any similar secretion. [<L, whey, watery fluid]