Samadhi is super-conscious state or state of liberation. It describes a non-dualistic state of consciousness in which the consciousness of the experiencing subject becomes one with the experienced object, and in which the mind becomes still, but the person remains conscious.

Samadhi is the main subject of the first part of the Yoga Sutras called Samadhi-pada. According to Vyasa, a major figure in Hinduism and one of the traditional authors of the Mahabharata, "yoga is samadhi." This is generally interpreted to mean that Samadhi is a state of complete control over the functions and distractions of consciousness.

In practice Samadhi is said to be the state of being aware of one’s existence without thinking, in a state of undifferentiated “Beingness." Three depths of Samadhi are usually understood in Hinduism - Laja Samadhi, Savikalpa Samadhi and Nirvikalpa Samadhi or Sahaja Samadhi.

Laja Samadhi

It is a latent, potential level of samadhi. It begins in deep meditation - even with movement such as dancing. This kind of samadhi is a state of joy, deep and general well-being, and peaceful meditation.

Savikalpa Samadhi

It is a initial temporary state of full-valued samadhi. The conscious mind is still active, as is the kalpa, meaning imagination. One should compare this meaning to that of sankalpa, which is 'wish.' Kalpa takes on a different, but related, meaning to sankalpa because one must use imagination or consciousness (kalpa) to envision a wish or desire (sankalpa). Conversely, vikalpa means "against imagination." At this final level of samadhi, the mind has become quiet and given up its desires and attendant. Vikalpa leads to the Truth, releasing one from any binds of mind. In Savikalpa Samadhi, we get the taste of bliss and beingness, but are still attached to our identification with the body as well as to our numerous worldly attractions.

Nirvikalpa Samadhi

It is the end state. There are no more kalpas (imaginings, wishes) because the mind is finally under control. Upon entering Nirvikalpa Samadhi, the differences we saw before have faded and we can see everything as one. In this condition pure awareness remains and nothing is missing to take away from wholeness and perfection. 

In the beginning, entering a samadhi requires lot of efforts and holding on to a state of samadhi takes even more effort. The beginning stages of samadhi are only temporary, by effort it is not meant that the mind has to work more. Instead, it means work to control the mind and release the self. Normal levels of meditation (mostly the lower levels) can be held automatically, as in "being in the state of meditation" rather than overtly "meditating." The ability to obtain positive results from meditation is much more difficult than simply meditating. It is recommended to find a qualified spiritual master (guru) who can teach a meditator about the workings of the mind.