When the mind is completely absorbed in one object of meditation, it is termed Samadhi. The mind identifies itself with the object of meditation. In Samadhi, there is neither meditation nor meditator. The meditator and meditated, the thinker and the thought, the worshipper and the worshipped become one or identical. The triad vanishes.
The mind loses its own consciousness and becomes identical with the object of meditation. The meditator has dissolved his personality in the sea of God, drowned and forgotten there till he becomes simply the instrument of God. When his mouth opens, it speaks Gods words without effort or forethought through direct intuition and, when he raises his hand, God flows again through that to work a miracle.
In Samadhi, there is neither seeing nor hearing. There is neither physical nor mental consciousness. There is only spiritual consciousness. There is only Existence (Sat). That is your real nature. When the water dries up in a pool, the reflection of the sun in the water also vanishes. When the mind melts in Supreme, when the mind-lake dries up, the reflected Chaitanya also vanishes. The Embodied Soul (personality) goes away. There remains Existence alone.
Turiya is the spiritual condition where there is no play of mind, where the mind is dissolved in Supreme. It is the fourth dimension, where there is infinite supreme bliss. It is not a condition of inertia, forgetfulness or annihilation. It is a state of absolute consciousness which baffles all attempts at description. It is the final goal of all. It is Mukti. It is Moksha or Nirvana.
Generally, when you have what you call dreamless sleep, it is one of two things; either you do not remember what you dreamt of or you fell into absolute unconsciousness which is almost death a taste of death. But, there is the possibility of a sleep in which you enter into an absolute silence, immortality and peace in all parts of your being and your consciousness merges into Satchidananda (Truth-Consciousness-Bliss). You can hardly call it sleep, for there is perfect awareness. In that condition, you can remain for a few minutes or hours or days; but, these few minutes give you more rest and refreshment than hours of ordinary sleep. You cannot have it by chance. It requires a long training.
Samadhi is not a stone-like inert state as many people imagine. A life in the spirit (Soul or Divine) is not annihilation. When the self is bound down to its empirical accidents, its activities are not fully exercised and, when the limitations of the empirical existence are transcended, the universal life is intensified and you have enrichment of Self. You will have a rich inner life. You will have an expanded cosmic life and supra-cosmic life, too.
Mind – Its Mysteries and Control by Sri Swami Sivananda