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SB 3.28.6

sva-dhiṣṇyānām eka-deśe
 manasā prāṇa-dhāraṇam
vaikuṇṭha-līlābhidhyānaṁ
 samādhānaṁ tathātmanaḥ
Synonyms: 
sva-dhiṣṇyānām — within the vital air circles; eka-deśe — in one spot; manasā — with the mind; prāṇa — the vital air; dhāraṇam — fixing; vaikuṇṭha-līlā — on the pastimes of the Supreme Personality of Godhead; abhidhyānam — concentration; samādhānam — samādhi; tathā — thus; ātmanaḥ — of the mind.
Translation: 
Fixing the vital air and the mind in one of the six circles of vital air circulation within the body, thus concentrating one’s mind on the transcendental pastimes of the Supreme Personality of Godhead, is called samādhi, or samādhāna, of the mind.
Purport: 

There are six circles of vital air circulation within the body. The first circle is within the belly, the second circle is in the area of the heart, the third is in the area of the lungs, the fourth is on the palate, the fifth is between the eyebrows, and the highest, the sixth circle, is above the brain. One has to fix his mind and the circulation of the vital air and thus think of the transcendental pastimes of the Supreme Lord. It is never mentioned that one should concentrate on the impersonal or void. It is clearly stated, vaikuṇṭha-līlā. Līlā means “pastimes.” Unless the Absolute Truth, the Personality of Godhead, has transcendental activities, where is the scope for thinking of these pastimes? It is through the processes of devotional service, chanting and hearing of the pastimes of the Supreme Personality of Godhead, that one can achieve this concentration. As described in the Śrīmad-Bhāgavatam, the Lord appears and disappears according to His relationships with different devotees. The Vedic literatures contain many narrations of the Lord’s pastimes, including the Battle of Kurukṣetra and historical facts relating to the life and precepts of devotees like Prahlāda Mahārāja, Dhruva Mahārāja and Ambarīṣa Mahārāja. One need only concentrate his mind on one such narration and become always absorbed in its thought. Then he will be in samādhi. Samādhi is not an artificial bodily state; it is the state achieved when the mind is virtually absorbed in thoughts of the Supreme Personality of Godhead.