plural serums or sera
ˈsirə 
1
medical
: the part of blood that is like water and that contains substances (called antibodies) that fight disease
blood serum
2
medical
: serum from an animal's blood that can be added to a person's blood to prevent or cure disease
The patient was administered serum.
mouse sera
3
biology
: the watery part of a fluid found in a plant
Cite this Entry
“Serum.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/serum. Accessed 16 Jun. 2026.
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