n. (pl. sera or serums) 1 a an amber-coloured liquid that separates from a clot when blood coagulates. b whey. 2 Med. blood serum (usu. from a non-human mammal) as an antitoxin or therapeutic agent, esp. in inoculation. 3 a watery fluid in animal bodies. serum sickness a reaction to an injection of serum, characterized by skin eruption, fever, etc.
[ L, = whey ]