Ramana Maharshi : The state in which the unbroken experience of existence-consciousness is attained by the still mind, alone is samadhi. That still mind which is adorned with the attainment of the limitless supreme Self, alone is the reality of God.
When the mind is in communion with the Self in darkness, it is called nidra [sleep], that is, the immersion of the mind in ignorance. Immersion in a conscious or wakeful state is called samadhi. Samadhi is continuous inherence in the Self in a waking state. Nidra or sleep is also inherence in the Self but in an unconscious state. In sahaja samadhi the communion is con-tinuous.
Question : What are kevala nirvikalpa samadhi and sahaja nirvikalpa samadhi?
Ramana Maharshi :The immersion of the mind in the Self, but without its destruction, is kevala nirvikalpa samadhi. In this state one is not free from vasanas and so one does not therefore attain mukti. Only after the vasanas have been destroyed can one attain liberation.
Question : When can one practise sahaja samadhi?
Ramana Maharshi : Even from the beginning. Even though one practises kevala nirvikalpa samadhi for years together, if one has not rooted out the vasanas one will not attain liberation.
Question : May I have a clear idea of the difference between savikalpa and nirvikalpa?
Ramana Maharshi : Holding on to the supreme state is samadhi. When it is with effort due to mental disturbances, it is savikalpa. When these disturbances are absent, it is nirvikalpa. Remaining permanently in the primal state without effort is sahaja.
Question : Is nirvikalpa samadhi absolutely necessary before the attainment of sahaja?
Ramana Maharshi : Abiding permanently in any of these samadhis, either savikalpa or nirvikatpa, is sahaja [the natural state]. What is body-consciousness? It is the insentient body plus consciousness. Both of these must lie in another consciousness which is absolute and unaffected and which remains as it always is, with or without the body-consciousness. What does it then matter whether the body-consciousness is lost or retained, provided one is holding on to that pure consciousness? Total absence of body-consciousness has the advantage of making the samadhi more intense, although it makes no difference to the knowledge of the supreme.
Source: from David Godman Excellent Book "Be As You are"