serum
Americannoun
plural
serums, sera-
the clear, pale-yellow liquid that separates from the clot in the coagulation of blood; blood serum.
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any watery animal fluid.
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the thin, clear part of the fluid of plants.
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milk whey.
noun
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See blood serum
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antitoxin obtained from the blood serum of immunized animals
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physiol zoology clear watery fluid, esp that exuded by serous membranes
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a less common word for whey
plural
serums-
See blood serum
-
Blood serum extracted from an animal that has immunity to a particular disease. The serum contains antibodies to one or more specific disease antigens, and when injected into humans or other animals, it can transfer immunity to those diseases.
Other Word Forms
Etymology
Origin of serum
1655–65; < Latin: whey
Explanation
Technically, serum is the part of blood that doesn’t clot, or any kind of “watery animal fluid.” But serum as skincare refers to watery fluid made of almost anything designed to make skin more beautiful. No animals involved. The medical term serum is used to describe the watery component of blood that's left behind when blood clots or coagulates. The serum of someone who's recovered from a disease is sometimes used to treat patients who fall ill with the same disease. Antibodies in the serum can fight off the virus or bacteria that caused the illness. In Latin, serum means "watery fluid." That’s what cosmetic companies have in mind when they sell various face serums.
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
In December, European-based Cosmo Pharmaceuticals reported results from a late-stage study of a serum that blocks the hormone driving hair loss directly at the scalp, without entering the bloodstream.
From The Wall Street Journal • May 1, 2026
Detecting such a strong signal from just 1 milliliter of serum was unexpected.
From Science Daily • Feb. 17, 2026
The rapid spread of that sticky serum tells you just how wildly successful K-beauty has become.
From BBC • Jan. 2, 2026
After years of being mistakenly linked to high serum cholesterol, eggs were rolled out to the public as a “healthy” food by the FDA External link in December 2024 External link.
From Barron's • Dec. 11, 2025
“If the Bureau is going to load all of those planes with the memory serum virus to reset the experiments, will there be any serum left to use against the compound?”
From "Allegiant" by Veronica Roth
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
