Samadhi is a state of mind, when the consciousness about him/her is lost and only objects remain. Steadiness and tranquillity of mind is essential for Samadhi which is achieved by observing eight limbs of self-discipline.
There is a difference between Sleep and Samadhi. In Samadhi manas (मन) is retired from the attachment of all sensual pleasures and is fully restrained. In sleep, manas is oblivious of the world but is not fully unattached to sensual pleasures. Sleep is tamsika. In Samadhi, manas is fully unattached to sensual pleasures.
Limbs of Self-Discipline:
The great Patanjali rishi prescribed eight limbs for self-discipline of Yoga to eliminate distractions of mind called as chitvritti viz. Yama (Self-restraints), Niyama (observances of rules), Asana (postures), Pranayama (regulation breath), Pratyhara (abstraction), Dharna (concentration), Dhyana (meditation), Samadhi (trance). (A separate detailed article on Patanjali yoga darshan is already published under the category of Philosophy)
Samadhi with open eyes:
Dispassion and steadiness of mind need to be achieved, while performing the worldly actions. In case, the mind is unsteady when the eyes are open, it is insentient Samadhi. Rishi Vishwamitra remained in Samadhi for sixty thousand years but when he opened the eyes, he fell in glamour of Menka.
Gopis used to go in state of Samadhi with open eyes. When Udhav ji explained the concept of Nirgun & Nirakar Brahman to Gopis and the need to meditate on it, Gopis said to Udhav ji that they see Shri Krishna everywhere and meditate him with open eyes. What is the need to meditate on Nirakar Brahman?
Types of Samadhi:
Samadhi is of two types’ viz. Samprajaatah Samadhi and Asamprajaatah Samadhi.
Samprajaatah or seed Samadhi: When distractions/modifications of mind are fully destroyed, this state is called Samprajaatah Samadhi or Samadhi with consciousness. Samprajaatah Samadhi is of four type’s viz. Savitarka Samadhi, Nirvitarka Samadhi, Savichar Samadhi and Nirvichar Samadhi.