A nude model
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 risqué and titillating. The Edo considered nudity a normal part of everyday life, however. (TNG: "Justice")
Betazoid wedding ceremonies are traditionally held in complete nudity, including the participants and the guests. ("Haven", et al.)
Tuvok slept in the nude and often dreamed 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")
See also
Background
Gene Roddenberry originally intended for his vision of Earth to be a civilization of nudists. (Star Trek is...)
According to the Star Trek: The Next Generation Companion, for the episode "Unnatural Selection", "the Darwin "superchildren" were originally to have appeared nude and the extras were asked to shave from the neck down. But the use of transparent furniture quickly nixed that idea, and costumes were hastily made as the extras endured stubble itch."