Webster's English Dictionary

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serum

se.rum \'sir-*m, 'ser-\ \-*\ n or serums or se.ra [L, whey, serum; akin to 
   Gk oros whey, serum, horme- onset, assa] pl  often attrib ult, Skt sarati 
   it flows 1: the watery portion of an animal fluid remaining after 
   coagulation :  1a: BLOOD SERUM; esp : immune blood serum that contains 
   speci fic immune bodies (as antitoxines or agglutinins) {antitoxin ~} 1b: 
   WHEY  1c: a normal or pathological serous fluid (as in a blister)  2: the 
   watery part of a plant fluid