serum

1. Blood serum used as a blood product, in the practice of medicine: blood serum from the tissues of immunized animals (human or nonhuman), containing antibodies and used to transfer immunity to another individual (human or nonhuman). (countable, uncountable) · Near-synonym: antiserum
2. A watery liquid from animal tissue, especially one that moistens the surface of serous membranes or that is exuded by such membranes when they become inflamed, such as in edema or a blister. (countable, uncountable)
3. The watery portion of certain other animal fluids, such as milk; whey. (countable, uncountable)
4. A skincare product of varying consistencies with a high concentration of one or more active ingredients, for a targeted purpose, such as preventing acne, repairing sun damage, or moisturizing. (countable, uncountable)

Plural serums