Hair
From Memory Alpha, the free Star Trek reference
Hair is a series of filaments of dead cells that grow on the skin of animals, usually mammals. In humanoids hair often grows on the top of the head, where it is styled for cultural reasons. On mammals, it was generally termed "fur".
The Federation Captain Jean-Luc Picard has been bald for most of his adult life.
Notably without hair, the Bolian race have sometimes been seen to wear a toupée to hide this fact. It is unknown if this is a fashion trend, or simply a way to blend in with most of the other humanoid species while away from their homeworld, Bolarus IX. (DS9: "The Muse")
When Seven of Nine was freed from the Borg Collective in 2374, The Doctor was able to stimulate her hair follicles to re-grow. (VOY: "The Gift") This was done at least once more, when, in 2376, Lyndsay Ballard returned to the crew of the USS Voyager as a Kobali. (VOY: "Ashes to Ashes")
In an alternate timeline, Data dyed half of his hair grey to look older. (TNG: "All Good Things...")
