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se·​rum ˈsir-əm How to pronounce serum (audio)
plural sera ˈsir-ə How to pronounce serum (audio) or serums
: the watery, clear portion of an animal fluid:
: the clear, yellowish fluid that remains from blood plasma after clotting factors (such as fibrinogen and prothrombin) have been removed by clot formation : blood serum compare antiserum, convalescent serum
: a normal or pathological serous fluid (as in a blister)
: whey
: 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
: the watery part of a plant fluid
Latex. This, the natural product of the rubber tree, is a suspension of rubber droplets in a watery serum concentrated and stabilized in such a way that the dry rubber content is between 60 and 70 per cent.C. A. O'Flaherty
see also truth serum


2 of 2


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

Examples of serum in a Sentence

Noun The patient was administered serum.
Recent Examples on the Web
This serum, which targets hyperpigmentation, is an undisputed standout. Perrie Samotin, Glamour, 29 Mar. 2024 After testing this serum, our skin texture appeared much softer and smoother with a more radiant finish. Jessie Quinn, Peoplemag, 25 Mar. 2024 Patients need to find a Quest Patient Center and have blood drawn, then the serum portion of the blood is analyzed by technology to provide the most accurate readings possible. Laura Schulte, Journal Sentinel, 5 Mar. 2024 When tamanu oil is incorporated into a formula — like the Kiehl's Daily Reviving Concentrate or Shea Moisture Tamanu Oil & Shea Butter Lotion — just follow the rules of application order for any serum or cream. Sophia Panych, Allure, 29 Feb. 2024 Ilia Beauty Skin Rewind Complexion Stick Imagine the Skin Rewind Complexion Stick as a middle ground between a foundation and concealer infused with the nourishing ingredients of your favorite serum. Alyssa Brascia, Peoplemag, 18 Mar. 2024 This serum is made with vitamin C and a powerful antioxidant complex that combats free radicals. Barbara Bellesi Zito, Travel + Leisure, 15 Mar. 2024 The super lightweight liquid feels like a serum but works like a concealer to instantly mask redness and dark circles sans any creasing, caking or cracking. Celia Shatzman, Forbes, 27 Feb. 2024 Other treatment serums can target common skin problems for men like acne, wrinkles, dark spots and dryness. Leslie Baumann, Miami Herald, 24 Feb. 2024
Lizzo completes her skincare process with a serum facial sheet mask from Cicapair. Charmaine Patterson, Peoplemag, 24 Oct. 2022

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'serum.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History



borrowed from Latin, "whey, wheylike fluid," of uncertain origin

Note: Latin serum has been conventionally linked with Greek orós, of identical meaning, and both compared with the Sanskrit verbal base sar-, allegedly meaning " flow" in Vedic texts (hence P. Chantraine in Dictionnaire étymologique de la langue grecque). More recently, however, this interpretation has been rejected, and sar- read as "run off"—hence Vedic Sanskrit ásarat "ran off," prasísarti "lets run, stretches, extends," not akin to sáraḥ "lake, pond" and related words, which describe still water. The Sanskrit etymon has been further compared with Greek hállomai "(I) spring, hop," Latin salīre "to jump" (see sally entry 2). If this is correct the Indo-European base is *sel-, and any connection to serum and orós impossible; these two words are left then without any certain etymology.


from attributive use of serum entry 1

First Known Use


1651, in the meaning defined at sense 1


1876, in the meaning defined above

Time Traveler
The first known use of serum was in 1651

Dictionary Entries Near serum

Cite this Entry

“Serum.” Dictionary, Merriam-Webster, Accessed 12 Apr. 2024.

Kids Definition


plural serums or sera

Medical Definition


1 of 2 noun
se·​rum ˈsir-əm How to pronounce serum (audio)
plural sera -ə How to pronounce serum (audio) or serums
: the watery portion of an animal fluid remaining after coagulation:
: 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

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


2 of 2 adjective
: occurring or found in the serum of the blood
serum cholesterol
serum glutamic-oxaloacetic transminase

More from Merriam-Webster on serum

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