Serum is the clear, yellowish liquid part of blood that remains after the blood cells and clotting proteins have been removed. When you get a cut and it stops bleeding, the watery fluid that might still seep out is similar to serum.

Medical researchers use serum all the time. When doctors want to test your blood for things like vitamin levels or signs of illness, they often examine your serum because it carries important substances throughout your body: nutrients, hormones, antibodies that fight disease, and waste products being carried away.

The word also appears in skincare, where a facial serum is a concentrated liquid treatment. These products borrowed the name because they're lightweight and absorb quickly into the skin.

You might hear about antivenom serum used to treat snake bites, or truth serum in spy movies (though real truth serums don't work like Hollywood suggests). In each case, the word suggests something potent and liquid that gets delivered into the body to create a specific effect.