serum

American  

[seer-uhm] / ˈsɪər əm /

noun

plural

serums, sera

1. the clear, pale-yellow liquid that separates from the clot in the coagulation of blood; blood serum.

2. immune serum.

3. any watery animal fluid.

4. the thin, clear part of the fluid of plants.

5. milk whey.

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serum British  

/ ˈsɪərəm /

noun

1. See blood serum

2. antitoxin obtained from the blood serum of immunized animals

3. physiol zoology clear watery fluid, esp that exuded by serous membranes

4. a less common word for whey

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serum Scientific  

/ sîr′əm /

plural

serums

1. See blood serum

2. 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.

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Other Word Forms

• serumal adjective

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

After confirming that the device could detect miR-21 in a controlled buffer solution, they tested it using human serum from lung cancer patients to simulate an actual blood test.

From Science Daily • Feb. 16, 2026

But for many people, trying to analyse the ingredients list on a bottle of moisturiser, cleanser or serum can be overwhelming.

From BBC • Dec. 13, 2025

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

As the YouTube hit demonstrates, hot sauce isn’t just for daredevils and masochists: It’s a truth serum.

From The Wall Street Journal • Nov. 20, 2025

Jahrling carried some slides containing spots of his own blood serum into his closet, shut the door, turned out the light.

From "The Hot Zone" by Richard Preston

Related Words

• antibody