Nudity refers to not having any clothes on. Most species must be naked to have sexual relations. Humans and Vulcans considered nudity to be somewhat risque and titillating. The Edo considered nudity a normal part of everyday life, however. (TNG: "Justice")
Tuvok slept in the nude and often dreampt of forgetting to put on his uniform before reporting for duty. (VOY: "Waking Moments")
A line in Henry V stated, "For though I speak it to you, I think the king is but a man, as I am. The violet smells to him as it doth to me, in his nakedness he appears but a man." (TNG: "The Defector")