A cuticle is the thin layer of dead skin at the base of your fingernails and toenails. If you look closely at where your nail meets your finger, you'll see a small rim of skin that overlaps the nail slightly. That's your cuticle, and it serves an important purpose: it seals the gap between your nail and skin, keeping out bacteria, dirt, and moisture that could cause infections.

Your cuticles protect the living cells underneath that constantly grow new nail material. Some people push back their cuticles or trim them during manicures to make their nails look neater, but doctors recommend being gentle, since damaging cuticles can let germs in.

The word cuticle can also refer to other protective outer layers in biology. Plants have a waxy cuticle covering their leaves that prevents water loss, much like how your nail cuticles protect the tender skin beneath. Insects have a tough cuticle that forms their exoskeleton. In each case, the cuticle is a protective barrier on the outside, shielding something more vulnerable underneath.