/ ˈsɪərəm; NAmE ˈsɪrəm/ noun ( pl. sera / -rə; NAmE / or ser·ums )
1.
[ U ] ( biology ) the thin liquid that remains from blood when the rest has clotted
2.
[ U , C ] ( medical ) serum taken from the blood of an animal and given to people to protect them from disease, poison, etc. :
snakebite serum
3.
[ U ] any liquid like water in body tissue
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WORD ORIGIN
late 17th cent.: from Latin , literally whey .