serum
Americannoun
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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
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See blood serum
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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
Derived 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.
The ASA also said that the serum was tested in a different climate to the UK, and may not work similarly.
From BBC • Apr. 28, 2026
Detecting such a strong signal from just 1 milliliter of serum was unexpected.
From Science Daily • Feb. 17, 2026
Pitt espouses a concept of simplicity in his men’s skincare routine — a three-step ritual starting with cleansing, serum to target signs of aging and then cream to lock in moisture.
From Los Angeles Times • Jan. 5, 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
Those lost in the memory serum haze are gathered into groups and given the truth: that human nature is complex, that all our genes are different, but neither damaged nor pure.
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.
