Skin is the external covering of a lifeform, consisting of a membranous tissue made up of skin cells. Skin is the largest body organ found on humanoids. The study of skin, in terms of its structure, function and skin-related diseases is known as dermatology.
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)
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)