INCI name
Niacinamide
Also listed as: Nicotinamide, vitamin B3 amide
Niacinamide is the amide form of vitamin B3. It dissolves in water and is used in serums and lotions. Makers add it for a range of claimed skin benefits; this site does not repeat marketing claims. What can be said plainly is that it is a well-studied vitamin derivative and a common ingredient in leave-on products.
Niacin (nicotinic acid) is a related molecule that can cause flushing. Niacinamide does not usually do that at the levels used in cosmetics, which is why the amide form is the one on skin-care labels.