serum

1 of 2

noun

se·​rum ˈsir-əm 

plural sera ˈsir-ə  or serums

1

: the watery, clear portion of an animal fluid:

a

: 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

b

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

c

: whey

2

: 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

3

: 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

serum

2 of 2

adjective

: 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

Noun

This is a featherweight serum that is a literal dream to apply. —Emily Orofino, Vogue, 30 Oct. 2025

Now, follow up your serum application with its accompanying moisturizer, and this is where the magic happens. —Katie Decker-Jacoby, StyleCaster, 29 Oct. 2025

Adjective

Lizzo completes her skincare process with a serum facial sheet mask from Cicapair. —Charmaine Patterson, Peoplemag, 24 Oct. 2022

Word History

Etymology

Noun

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.

Adjective

from attributive use of serum entry 1

First Known Use

Noun

1651, in the meaning defined at sense 1

Adjective

1876, in the meaning defined above

Phrases Containing serum

• blood serum
• convalescent serum
• serum albumin
• serum globulin
• serum hepatitis
• serum sickness
• truth serum

Rhymes for serum

• theorem
• thiram
• antiserum
• variorum

Kids Definition

serum

noun

se·​rum

ˈsir-əm

plural serums or sera

ˈsir-ə

1

: blood serum

2

: antiserum

Medical Definition

serum

1 of 2 noun

se·​rum ˈsir-əm 

plural sera -ə  or serums

: 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)

serum

2 of 2 adjective

: occurring or found in the serum of the blood

serum cholesterol

serum glutamic-oxaloacetic transminase