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

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 BBC visited a sprawling Amorepacific factory outside South Korea's capital Seoul, where a handful of workers oversaw fully automated production lines bottling Laneige's Water Sleeping Mask and CosRX's Vitamin C 23 Serum.

From BBC • Jan. 2, 2026

She got the tattoos from Levi Elorreaga, an L.A. artist who was doing a residency at Black Serum, a studio in San Francisco at the time.

From Los Angeles Times • Nov. 3, 2025

Department of Defense Serum Repository, which stores blood from military applicants.

From Science Daily • Oct. 21, 2025

Newt yelled for the Grief Serum; a minute later Thomas felt a pinprick on his arm.

From "The Maze Runner" by James Dashner

Related Words

• antibody