(covers information from several alternate timelines)
A nail was a covering of keratin on the end of one's digits (fingers or toes).
Fingernail
According to Doctor Phlox, germs can survive everywhere, also 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")
Toenail
Denobulans had colored long toenails. (ENT: "Bounty")
See also