The word "Khalsa" is derived from Arabic "Khalis" means ('pure' or 'unsullied').
"Khalsa" name is given by Guru Gobind Singh Ji which means 'pure', and this name is given to all sikhs who have been "Baptised"(means Amritdhari) by taking 'Amrit' in a ceremony that is called "Amrit Sanchar".
The first time this ceremony fell on 30 march, 1699 at Anandpur Sahib in Punjab (India).
A Khalsa always carry these five symbols, that are Kanga, Kachchera, Kara, Kesh, Kirpan these all five symbols, known as 'Panj Kakkea' (Kakkar) and very must for a Khalsa.
The word "Force". It means, 'Strenght or Energy as an attribute of physical action or Movement', so khalsa was also used 'Major Military Force ', and one of the major duties of a khalsa was to protect the weak and the helpless because that time the rising torture of the Mughal rules, so Khalsa had weapons with them. They were well trained in Martial arts, even sikh needed protection because of the rising torture of Mughal rules. So, Khalsa were appionted for this purpose. The Khalsa Force is also known as "Akal Purakh Ki Foj"