Serum has two primary meanings depending on the context: 1. In blood and medical context:
Serum is the clear, yellowish fluid component of blood that remains after blood has clotted. It is essentially blood plasma without the clotting factors such as fibrinogen. Serum contains proteins (except clotting factors), electrolytes, antibodies, antigens, hormones, and other substances like drugs or microorganisms. It is obtained by allowing blood to clot and then centrifuging to remove the clot and blood cells. Serum is used in diagnostic tests, blood typing, and immunotherapy, such as using convalescent serum containing antibodies from recovered patients to treat others

. 2. In skincare:
Serum refers to a lightweight, highly concentrated skincare product designed to deliver active ingredients deeply into the skin. These products have smaller molecules than typical moisturizers, allowing them to penetrate deeply and target specific skin concerns like wrinkles, dryness, discoloration, or redness. Serums are applied after cleansing and before moisturizing and often contain ingredients like hyaluronic acid, vitamin C, or niacinamide to improve skin health and appearance

. In summary, serum can mean either the blood fluid without clotting factors used in medical diagnostics and treatments or a potent skincare product for targeted skin care benefits.