The eponychium is the living skin at the base of the nail that covers the proximal nail fold and produces the cuticle, forming the visible border between finger skin and nail plate.
Also known as: proximal nail fold skin, cuticle (common usage), nail fold, eponychial fold
The eponychium is the fold of living skin at the base of the nail, the part that most people call the cuticle. It lies over the proximal nail fold, which covers the matrix. The eponychium sheds a thin layer of dead cells onto the nail plate as the plate grows out. That dead tissue, stuck to the plate, is the true cuticle (Medscape).
The distinction matters in the salon and at home. The cuticle, being dead, can be softened and pushed back. The eponychium, being living skin, should not be cut. Cutting it opens the seal over the matrix, invites infection and makes the skin grow back thicker and more ragged. Trimming is reserved for hangnails, the small splits of skin at the sides of the nail (Essie).
Under the eponychium, the roof of the nail fold gives the plate its natural shine. Damage to this area, from picking, biting or rough pushing, can show as a dull or ridged patch in the nail weeks later.
Care is the same as for the cuticle: oil daily with jojoba, sweet almond or vitamin E oil, gloves for wet work, and an occlusive balm overnight in winter (Today). Redness, swelling or pus around the eponychium is paronychia, an infection of the nail fold, and is a reason to see a GP.