What Is a Serum? Serum, Essence and Ampoule, Told Apart
A light, usually water based product that carries the ingredients you buy it for, applied after cleansing and before moisturiser.
On the label
A serum can be anything from plain glycerin and water to a heavily loaded active. The word promises a texture, not a result.
What it does not mean
It does not replace a moisturiser for dry skin, and it does not need to tingle to be working.