n The clear yellowish fluid obtained upon separating whole blood into its solid and liquid components after it has been allowed to clot. Also called blood serum.
n Blood serum from the tissues of immunized animals, containing antibodies and used to transfer immunity to another individual, called antiserum.
n A watery fluid 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.
n The watery portion of certain animal fluids, as blood, milk, etc; whey.
n (context|skincare) An intensive moisturising product to be applied after cleansing but before a general moisturiser.