Sikhism is a religion practiced by Human civilization on Earth. The religion was founded in the 16th century in northern India. Followers of the religion are known as Sikhs.
Khan Noonien Singh was of Sikh descent. Khan's last name of Singh was the traditional name given to Sikh males. As was traditional amongst the Sikh, he did not cut his hair nor shave his beard as a young man. This changed when he reached his 21st birthday, and decided to no longer follow tradition. After Khan was rescued by the Federation starship USS Enterprise in the year 2267, ship's historian Marla McGivers was the first to identify Khan as a Sikh. (TOS episode & Star Trek 2 novelization: Space Seed, ST - The Eugenics Wars novel: The Rise and Fall of Khan Noonien Singh, Volume 2)