Privatisation means transferring the control of an enterprise from the government sector to the private sector. Generally, but not always, this also means transferring ownership of the Public sector enterprise as well as control.
It can be accomplished by sale or lease. It can be accomplished by the government selling 100% of an enterprise, or selling 51%, or even by selling a minority stake – so long as the private sector is given full managerial control. Without transferring control to the private sector, the government can rise money by selling a smaller share, but that is not privatisation as such.