an amber, watery fluid, rich in proteins, that separates out when blood coagulates
"After letting the sample stand until it clotted, a clear yellow serum separated from the solid clot."
The clear yellowish liquid obtained upon separating whole blood into its solid and liquid components after it has been allowed to clot.
"The doctor ordered a test on the serum to check for signs of infection without needing any additional anticoagulants."
In plain English: Serum is the clear liquid part of blood that remains after all the cells and clotting proteins have been removed.
"The doctor recommended that she take daily vitamin serum for her skin health."
Usage: Serum is the fluid remaining in blood after clotted cells are removed, distinguishing it from plasma which contains fibrinogen. Use this term specifically for medical or biological contexts involving separated blood rather than general liquids like milk serum found on cheese labels.
The word serum comes from the Latin term serum, which originally meant "whey," the liquid part left after milk has been curdled and strained into cheese. It entered English directly from this classical source to describe a similar clear fluid found in blood or other bodily tissues.