A Sikh (pronounced "Sick-h") was a follower of the Earth religion known as Sikhism. Sikhism could be symbolized by a khanda.
After the red angel transported a group of Humans in the East Fork Presbyterian Church from the bombings of World War III in 2053 to the distant planet Terralysium, the "New Eden" settlers "cobbled" together diverse faiths – including Sikhism. A khanda representing Sikhism was a component of the stained glass windows in the White Church. (DIS: "New Eden")
Upon first viewing Khan Noonien Singh in 2267, USS Enterprise historian Lieutenant Marla McGivers visually identified him as being "from the northern India area", and further indicated that he was "probably a Sikh," a people whom she described as having been "the most fantastic warriors." (TOS: "Space Seed")
In 2380, Lieutenant Arjun of the USS Cerritos was Sikh and wore a turban. (LD: "Second Contact")