A nail was a covering of keratin on the end of one's digits (fingers or toes).
Fingernail[]
According to Doctor Phlox, germs could survive almost anywhere, even under a fingernail. (ENT: "Breaking the Ice")
In 2259 of the alternate reality, Carol Marcus wore polish on her fingernails that matched the color of her sciences division uniform. (Star Trek Into Darkness)
In 2269, while inhabiting the body of James T. Kirk, Janice Lester filed his fingernails. (TOS: "Turnabout Intruder")
A microscopic generator hidden under his fingernail enabled Rao Vantika to transfer his consciousness to Doctor Julian Bashir in 2369. (DS9: "The Passenger")
In 2371, Jadzia Dax chided her former fellow host of the Dax symbiont Tobin Dax for biting Chief Miles O'Brien's nails while speaking through his body for Jadzia's zhian'tara. Tobin apologized, saying he was nervous. (DS9: "Facets")
A recurring continuity problem during the first two seasons of Star Trek: The Next Generation were Gates McFadden's fingernails. While her character Doctor Beverly Crusher had short nails in the scenes, her hand double had long nails in the close-up shots in episodes such as "When The Bough Breaks".
Toenail[]
Denobulan toenails
Denobulans had colored long toenails. (ENT: "Bounty")
Medical conditions[]