serum /sir″əm/ pl. sera [L, whey] , 1. the fluid portion of blood that remains subsequent to in vitro clotting. Unlike plasma, serum lacks fibrinogen and several of the coagulation proteins. 2. any clear watery fluid that has been separated from its more solid elements, such as the exudate from a blister. 3. a vaccine or toxoid prepared from the serum of a hyperimmune donor for prophylaxis against a particular infection or poison.
