A Sikh is a follower of Sikhism, a monotheistic religion that originates during the fifteenth century in the Punjab region of the Indian subcontinent.

The term "Sikh" has its origin in the Sanskrit words शिष्य (śiṣya; disciple, student) or शिक्ष (śikṣa; instruction).

A Sikh, according to Article I of the Sikh Rehat Maryada (the Sikh code of conduct), is "any human being who faithfully believes in One Immortal Being; ten Gurus, from Guru Nanak to Guru Gobind Singh; Guru Granth Sahib; the teachings of the ten Gurus and the baptism bequeathed by the tenth Guru".

"Sikh" properly refers to adherents of Sikhism as a religion, not an ethnic group.

However, because Sikhism has seldom sought converts, most Sikhs share strong ethno-religious ties.

Many countries, such as the United Kingdom, therefore recognize Sikh as a designated ethnicity on their censuses.

The American nonprofit organization United Sikhs has fought to have Sikh included on the U.S. census as well, arguing that Sikhs "self-identify as an 'ethnic minority'" and believe "that they are more than just a religion".

Male Sikhs have "Singh" (Lion), and female Sikhs have "Kaur" (princess) as their middle or last name.

Sikhs who have undergone the khanḍe-kī-pahul (the Sikh initiation ceremony) may also be recognized by the five Ks: Kesh uncut hair which is kept covered, usually by a turban to protect the Dasam Duwar ("god head"); an iron or steel bracelet (kara); a kirpan (a sword tucked into a gatra strap or a kamal kasar belt); kachehra, a cotton undergarment, and kanga, a small wooden comb.

Initiated male and female Sikhs must cover their hair with a turban.

The greater Punjab region is the historic homeland of the Sikhs, although significant communities exist around the world.