The word “Samadhi” is used to mean trance as a practice, as a technique, being one of the eight Angas of the Ashtanga Yoga of Patanjali. It is a Sadhana; it is something that is practised. But, quite often, the same word “Samadhi” is also used in the sense of the culmination of Yoga or the ultimate objective or goal of Yoga; then it denotes a state of superconsciousness, a state transcending all mind-activity. Therefore, it will not be wrong to say that in the Yoga Shastra the term as such is used to mean both a practice as well as a state of superconsciousness. As a practice or a technique, it is referred to by various names as Asmita Samadhi, Savitarka Samadhi, Savichara Samadhi and so on. In English sometimes, writers have been in the practice of alluding to these as the lower Samadhis. When the Arurukshu Yogi—a Yogi who has already climbed sufficiently well up on the ladder of Yoga and has reached a very high state—goes on practising Samadhi, diligently and with great exertion, without giving up, without tiring, with sustained zeal, with Vairagya and great regularity, with great tenacity of purpose, for months and years, then he ascends into higher and higher states of Samadhi. The Samadhi in such a high state is referred to as Nirbija Samadhi or Nirvikalpa Samadhi or Asamprajnata Samadhi. When the word “Samadhi” is used in this way to refer to the Nirbija Samadhi or Asamprajnata Samadhi, then it means the superconscious state. They even go so far as to say that it is a state of non-dual consciousness. That is a matter of opinion. When the term “Samadhi” is used to indicate the Savitarka Samadhi or the Savikalpa Samadhi or other lower Samadhis, then it means trance which is a technique and a practice. When one reaches the level of the Asamprajnata Samadhi or the Nirvikalpa or Nirbija Samadhi, sometimes the Yogi goes on practising such a state until he becomes so much established in that state of consciousness that even when he comes back into the waking state, down from the deep inward state, where he is not aware of the body or the time or the surroundings, even when he comes back into the normal state, his awareness continues to be qualified by the same state of non-duality. In other words, he is so much established in that state of spiritual consciousness or awareness that even while he is moving and acting, he still remains in that state of inner awareness, and they call this the state of Sahaja Samadhi. Sahaja means natural. So, in Sahaja Samadhi, the state of non-dual consciousness becomes to the Yogi his natural state, and not a state which he tries hard to reach and then reaches only to come back to the waking state after a while. Rather, the state of non-dual consciousness becomes normal to him. The Yogi thus gets established in Sahaja Avastha. But, the Sahaja Avastha is a rare phenomenon and is itself the fruit of intense practice of the other stages and gradations of Samadhi. It is only after intense practice of Savitarka Samadhi, Savichara Samadhi and Asmita Samadhi and the continued practice of being in a state of Nirvikalpa Samadhi that the Nirvikalpa Samadhi becomes natural to the Yogi, that it becomes continued and unbroken in all the three states, namely, waking, dream and deep sleep. Thus, in the Sahaja Avastha, even in the waking state, even in the midst of activity, the Yogi rests in non-dual consciousness.
Source: THE PHILOSOPHY, PSYCHOLOGY AND PRACTICE OF YOGA, Chapter 16