Know how to use "fewer" and "less"? Find out.
1670s, "watery animal fluid," from Latin serum "watery fluid, whey," from PIE root *ser- (2) "to run, flow" (cf. Greek oros "whey;" Sanskrit sarah "flowing," sarit "brook, river"). First applied 1893 to blood serum used in medical treatments.
serum se·rum (sēr'əm)
n. pl. se·rums or se·ra (sēr'ə)
A watery fluid, especially one that moistens the surface of serous membranes or that is exuded by such membranes when they become inflamed.
The clear yellowish fluid obtained upon separating whole blood into its solid and liquid components.
Such fluid from the tissues of immunized animals, containing antibodies and used to transfer immunity to another individual.
Plural serums or sera
the portion of plasma remaining after coagulation of blood, during which process the plasma protein fibrinogen is converted to fibrin and remains behind in the clot. Antiserum, which is prepared from the blood of animals or humans that have been exposed to a disease and have developed specific antibodies, is used to protect persons against disease to which they have been exposed.