Khalsa which means 'pure' is the name given by Guru Gobind Singh to all Sikhs who have been baptised or initiated by taking Amrit in a ceremony called Amrit Sanchar. The first time that this ceremony took place was on Baisakhi, which fell on 30 March 1699 at Anandpur Sahib in Punjab, India. The Sikhs celebrated the 300th anniversary of the day in 1999 with thousands of religious gatherings all over the world.
The word "Khalsa" is derived from Arabic khalis (literally meaning "pure" or "unsullied") and Perso-Arabic khalisah (literally pure; office of revenue department; lands directly under government management), is used collectively for the community of baptised Sikhs. The term khalisah was used during the Muslim rule in India for crown-lands administered directly by the king without the mediation of jagirdars or mansabdars.
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Kaho Kabīr jan bẖaė kẖālsė parėm bẖagaṯ jih jānī. ॥4॥3॥
In the Sikh tradition, the term appears again in one of the hukamnamas (literally written order or epistle) of Guru Hargobind (1595-1644) where a sangat of the eastern region has been described as "Guru ka Khalsa" (Guru's own or Guru's special charge). It has also been employed in the same sense in one of the letters of Guru Tegh Bahadur (1621-75) addressed to the sangat of Patna. The word occurs in Sikh Scripture, the Guru Granth Sahib, once, but there it carries the sense of the term khalis, i.e. pure.(see below)
The term "Khalsa", however, acquired a specific connotation after Guru Gobind Singh (1666-1708) introduced, on 30 March 1699, the new form of initiatory rites— khande di pahul (rites by khanda or double-edged sword). Sikhs so initiated on that Baisakhi day were collectively designated as the Khalsa — Khalsa who belonged to Vahiguru, the Supreme Lord. The phrase Vahiguru ji ka Khalsa became part of the Sikh salutation: Vahiguru ji ka Khalsa, Vahiguru ji ki Fateh (Hail the Khalsa who belongs to the Lord God! Hail the Lord God to whom belongs the victory!!)