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
Inflected Forms
Nouns
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 Serum Institute of India is lined up to mass produce the Ebola vaccine once Oxford can supply medical-grade material.
From BBC • May 22, 2026
The San Francisco phone booth is set up outside of Black Serum Tattoo studio in the Mission District.
From Los Angeles Times • Jan. 22, 2026
They are “two very different countries,” Dr. Anders Hviid, leader of vaccine safety and effectiveness research at the Statens Serum Institut in Denmark, told CNN.
From Salon • Jan. 6, 2026
The breaking point came after Chanel released an antiaging product called Blue Serum, advertised as being made from ingredients from the blue zones: green coffee from Costa Rica, olives from Sardinia, and mastic from Greece.
From Science Magazine • Nov. 20, 2024
Nah, just makes me think the Grief Serum and the Changing got somethin’ fishy about ’em.
From "The Maze Runner" by James Dashner
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.
