The word Samadhi essentially means intuitive knowledge of the highest state. There are a multitude of classifications of Samadhi, but they can be broken down into two fundamental streams or conditions: Savikalpa and Nirvikalpa. Savikalpa means with form, with identity, with thought. Nirvikalpa means without form, without identity, without thought.
There are many stages of attention within both of these states, but what is fundamentally taking place is the contact of the individual identity with the universal identity, the substance of the infinite creation. The drop of water is dropped into the ocean; the totality of the ocean is in the drop, and the totality of the drop is in the ocean. The platform of identity moves from a point of individual and separate self to the utterly merged and commingled Self.
This is something that occurs naturally at the event we call death. The dropping of the shell of individual self—the body, life force, and mental formation—leads to a revelation of totality and utter independence. This experience is so profound, so dramatic that for the unprepared mind—one unconditioned by yoga, meditation, and the conditions of Samadhi—it simply goes white noise at the end of each life as the mind collapses in the face of totality.