Sri Ramana Maharishi

Sri Ramana Maharshi  explains SAMADHI  in the following way: 

(1) Holding on to Reality is samAdhi. 

(2) Holding on to Reality 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. 
(from "Talk 391") 

He further explains what He means by the difference between (3) and (5) in another talk: 

Even if one is immersed in nirvikalpa samAdhi for years together, when he emerges from it he will find himself in the environment which he is bound to have. That is the reason for the AchArya emphasising sahaja samAdhi in preference to nirvikalpa samAdhi in his excellent work vivekachUDAmaNi. One should be in spontaneous samAdhi - that is, in one's pristine state - in the midst of every environment. (Talk 54) 

and again in Talk 187, Sri Ramana explains both (3) and (5) as nirvikalpa samAdhi, but the sahaja type alone is permanent: 

In sleep the mind is alive but merged in oblivion (see (4) above). - In kevala nirvikalpa samAdhi, the mind is alive but merged in light, like a bucket with rope lowered into a well, that can be drawn out again. - In sahaja nirvikalpa samAdhi, the mind is dead , resolved into the Self, like a river discharged into the ocean - its identity lost - and which can never be re-directed from the ocean, once discharged into it.
(Talk 187) 

Thus when the disciple asks, "Is loss of body-consciousness a perquisite to the attainment of sahaja samAdhi?" Sri Ramana replies: 

What is body-consciousness? Analyse it. There must be a body and consciousness limited to it which together make up body-consciousness. These must lie in another Consciousness which is absolute and unaffected. Hold it. That is samAdhi. It exists when there is no body-consciousness because it transcends the latter, it also exists when there is the body-consciousness. So it is always there. What does it matter whether body-consciousness is lost or retained? When lost it is internal samAdhi: when retained, it is external samAdhi. That is all. A person must remain in any of the six samAdhi-s so that sahaja samAdhi may be easy for him.
(Talk 406) 

Below, Sri Ramana refers to samAdhi, not as a state to be gained, as in some of the many different types but as our natural state. He also points out the importance of the waking state, which I believe was a query in another thread: 

Samadhi is one's natural state. It is the under-current in all the three states. This - that is, 'I' - is not in those states, but these states are in It. If we get samAdhi in our waking state that will persist in deep sleep also. The distinction between consciousness and unconsciousness belongs to the realm of mind, which is transcended by the state of the Real Self.

(Talk 136) 

By shravaNa, Knowledge dawns. That is the flame. By manana, the Knowledge is not allowed to vanish. Just as the flame is protected by a wind-screen, so the other thoughts are not allowed to overwhelm the right knowledge. By nididhyAsana, the flame is kept up to burn bright by trimming the wick. Whenever other thoughts arise, the mind is turned inward to the light of true knowledge. When this becomes natural, it is samAdhi. The enquiry "Who am I?" is the shravaNa. The ascertainment of the true import of 'I' is the manana. The practical application on each occasion is nididhyAsana. Being as 'I' is samAdhi.
(Talk 647) 

Eternal, unbroken, natural state is jnAna." 
(Talk 385)