1.Holding on to Reality is samadhi.
2.Holding on to samadhi with effort is savikalpa samadhi.
3.Merging in Reality and remaining unaware of the world is nirvikalpa samadhi.
4.Merging in ignorance and remaining unaware of the world is sleep.
5.Remaining in the primal, pure, natural state without effort is sahaja nirvikalpa samadhi
(Ramana Maharshi (1879-1950))