Skin was the external covering of a lifeform, consisting of a membranous tissue made up of skin cells, composed of two membranes, the upper of which was known as the epidermis or epidermal tissue. Skin was the largest bodily organ found on humanoids. The study of skin – in terms of its structure, function, and skin-related diseases – was known as dermatology. Parts of the body which were in direct connection with skin were hair and the nervous system.
Skin color is often a distinguishable trait between different alien species and has been the focal point of slang terms, such as pink skin or "monotone", to more extreme xenophobic actions, such as war; one such skin-color-driven war destroyed the population of Cheron. (TOS: "Let That Be Your Last Battlefield"; ENT: "The Andorian Incident", et al)
In 2267, Spock reported to Captain James T. Kirk, while Lieutenant Hikaru Sulu and Yeoman Tonia Barrows were also present, that the face of a Black Knight which essentially seemed to be a dummy wasn't Human skin tissue, since the knight was actually a robot. (TOS: "Shore Leave")
In 2370, Talur noted that Data's skin was clammy. She attributed his strange skin coloration to exposure to cold temperatures in the Vellorian Mountains. Later, Data took skin samples from Gia and Garvin to determine the cause of the Barkon IV villagers' mysterious illness. (TNG: "Thine Own Self")
After traveling back in time to the year 2063, the Borg Queen grafted skin onto Data's android frame in an attempt to make him more susceptible to the Borg Collective's seduction. (Star Trek: First Contact)