se·​rum | \ˈsir-əm \
plural sera\ˈsir-​ə \ or serums

Definition of serum 

(Entry 1 of 2)

1 : the watery portion of an animal fluid remaining after coagulation:

a(1) : blood serum

(2) : antiserum

b : whey

c : a normal or pathological serous fluid (as in a blister)

2 : the watery part of a plant fluid

3 : a usually lightweight cosmetic preparation especially for use on the face specifically : a typically water-based, often concentrated preparation that lacks lubricating and thickening agents an antiaging facial serum



Definition of serum (Entry 2 of 2)

: occurring or found in the blood serum serum cholesterol serum antibodies

Examples of serum in a Sentence


The patient was administered serum.

Recent Examples on the Web: Noun

Not only are there creams, masks, and serums taking over store shelves, but supplements also consistently claim to reverse the visible signs of aging. Amy Capetta, Woman's Day, "5 Anti-Aging Supplements That May Actually Work," 16 Oct. 2018 The hyaluronic [shown] and super anti-aging serums. Fiorella Valdesolo, WSJ, "3 Skincare Experts on Their Favorite Things," 23 Aug. 2018 Studies in mice also indicate that paternal malnutrition can lead to elevated serum glucose and thus hypertension—which can lead to cerebral hemorrhage—in male offspring only. Diana Gitig, Ars Technica, "Trauma of US Civil War POW experience affected the next generation," 22 Oct. 2018 Add a few drops to a serum or moisturizer, or pat it directly on your skin every day. Ashley Weatherford, The Cut, "Everything at Paula’s Choice Is on Sale Right Now," 13 June 2018 Owners of brands such as DankGals (CBD and floral bath soaks), Kana Skincare (lavender and CBD sleep masks) and Ambika Herbals (serums and scrubs) will be on hand to answer questions about how to use the products and what to expect from them. Kavita Daswani,, "Luxury Garage Sale brings vintage pieces to Westfield pop-ups, Nanushka offers Eastern European styles at Platform," 10 May 2018 Her collection of products will include everything like eye cream, moisturizer, neck cream, serum and more in addition to her special Unicorn Mist, which is available now on the brand’s website. Kaitlyn Frey,, "Paris Hilton Finally Found Her Wedding Gown and It's 'So Iconic'," 23 Apr. 2018 Ultimately, all of the compounds and technology create a serum-like product that sinks into skin, protects it, and gives a dewy finish and fresh feel. Shannon Barbour, The Cut, "This Skin-Care Product Works on a Cellular Level," 24 May 2018 Kevyn Aucoin’s Sensual Skin Fluid Foundation, which conceals scarring and redness with its serum-like texture, is also on sale for $44, on sale from $65. Lauren Swanson, Allure, "Dermstore Is Having a Secret Sale That You Need to Know About," 13 Apr. 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'serum.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of serum


1665, in the meaning defined at sense 1


1876, in the meaning defined above

History and Etymology for serum


Latin, whey, wheylike fluid; akin to Greek oros whey

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Statistics for serum

Last Updated

6 Dec 2018

Look-up Popularity

Time Traveler for serum

The first known use of serum was in 1665

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English Language Learners Definition of serum

medical : the part of blood that is like water and that contains substances (called antibodies) that fight disease

medical : serum from an animal's blood that can be added to a person's blood to prevent or cure disease

biology : the watery part of a fluid found in a plant


se·​rum | \ˈsir-əm \

Kids Definition of serum

: the clear liquid part that can be separated from coagulated blood and contains antibodies


se·​rum | \ˈsir-əm \
plural sera\-​ə \ or serums

Medical Definition of serum 

(Entry 1 of 2)

: the watery portion of an animal fluid remaining after coagulation:

a(1) : the clear yellowish fluid that remains from blood plasma after fibrinogen, prothrombin, and other clotting factors have been removed by clot formation

called also blood serum

(2) : antiserum

b : a normal or pathological serous fluid (as in a blister)



Medical Definition of serum (Entry 2 of 2)

: occurring or found in the serum of the blood serum cholesterol serum glutamic-oxaloacetic transminase

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More from Merriam-Webster on serum

Rhyming Dictionary: Words that rhyme with serum

Spanish Central: Translation of serum

Nglish: Translation of serum for Spanish Speakers

Britannica English: Translation of serum for Arabic Speakers

Comments on serum

What made you want to look up serum? Please tell us where you read or heard it (including the quote, if possible).


to make faulty or ineffective

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