Diazolidinyl and Imidazolidinyl Urea
A preservative which is a strong skin irritant. In aqueous solutions it can release formaldehyde.
It also contains ammoniums salts and may be contaminated with large amounts of toxins in the manufacturing process.
As a skin irritant it is also Acne producing, mutagenic and carcinogenic.
By slowing down moisture loss it extends product shelf-life which is one of the main reasons it is heavily used by the larger cosmetic companies.
It also assists absorption of other ingredients deeper into the skin- unfortunately both good and bad ingredients, so you really don't want it present in a product that contains other questionable ingredients.