Samādhi means "fixed mind." The Vedic dictionary, the Nirukti, says, samyag ādhīyate 'sminn ātma-tattva-yāthātmyam: "When the mind is fixed for understanding the self, it is said to be in samādhi. " Samādhi is never possible for persons interested in material sense enjoyments and bewildered by such temporary things. They are more or less condemned by the process of material energy.
(Bhagavad-Gita--2:44--purport).

To say that one is in samādhi is to say that one has fully realized Kṛṣṇa consciousness; that is, one in fullsamādhi has realized Brahman, Paramātmā and Bhagavān. The highest perfection of self-realization is to understand that one is eternally the servitor of Kṛṣṇa and that one's only business is to discharge one's duties in Kṛṣṇa consciousness. A Kṛṣṇa conscious person, or unflinching devotee of the Lord, should not be disturbed by the flowery language of the Vedas nor be engaged in fruitive activities for promotion to the heavenly kingdom. In Kṛṣṇa consciousness, one comes directly into communion with Kṛṣṇa, and thus all directions from Kṛṣṇa may be understood in that transcendental state. One is sure to achieve results by such activities and attain conclusive knowledge. One has only to carry out the orders of Kṛṣṇa or His representative, the spiritual master.
(Bhagavad-Gita--2:53---purport).

A pure devotee of the Lord constantly sees the presence of the Lord within himself because of being transcendentally related by loving service. Such a pure devotee cannot forget the Lord for a moment. This is called trance. The mystic (yogī) tries to concentrate upon the Supersoul by controlling the senses from all other engagements, and thus he ultimately attains samādhi. A devotee more easily attains samādhi, or trance, by constantly remembering the Lord's personal feature along with His holy name, fame, pastimes, etc. Therefore, the concentration of the mystic yogī and that of the devotee are not on the same level. The concentration of the mystic is mechanical, whereas that of the pure devotee is natural in pure love and spontaneous affection.
(Srimad Bhagavatam---1:9:39---purport).

This fixed meditation is called samādhi, or trance. A person constantly engaged in the transcendental loving service of the Lord cannot be deviated from meditating on the form of the Lord, as described herein. The arcana-mārga, or the devotional path prescribed in the Pañcarātra system of devotional service for worshiping the Deity in the temple, makes the devotee think constantly of the Lord; that is samādhi, or trance. One who practices in this way cannot deviate from the service of the Lord, and that makes him perfect in the mission of human life.
(Srimad Bhagavatam---4:8:52---purport).

Recollection, absorption, meditation, constant remembrance and trance are the five items of progressive kṛṣṇa-smaraṇa. At first, remembrance of Kṛṣṇa may be interrupted at intervals, but later remembrance proceeds uninterrupted. When remembrance is uninterrupted, it becomes concentrated and is called meditation. When meditation expands and becomes constant, it is called anusmṛti. By uninterrupted and unceasing anusmṛti one enters the stage of samādhi, or spiritual trance. After smaraṇa-daśā orsamādhi has fully developed, the soul comes to understand his original constitutional position. At that time he can perfectly and clearly understand his eternal relationship with Kṛṣṇa. That is called sampatti-daśā, the perfection of life.
(Nectar of Instruction).

Therefore, as soon as one's mind is controlled through one of the yoga systems, one should be considered to have already reached the destination. One has to abide by superior dictation. When one's mind is fixed on the superior nature, he has no alternative but to follow the dictation of the Supreme. The mind must admit some superior dictation and follow it. The effect of controlling the mind is that one automatically follows the dictation of the Paramātmā, or Supersoul. Because this transcendental position is at once achieved by one who is in Kṛṣṇa consciousness, the devotee of the Lord is unaffected by the dualities of material existence, namely distress and happiness, cold and heat, etc. This state is practical samādhi, or absorption in the Supreme.
(Bhagavad-Gita---6:7---purport).

Either in waking or in sleep, a devotee of the Lord can never be free from thinking of Kṛṣṇa's name, form, qualities and pastimes. Thus a devotee of Kṛṣṇa can conquer both sleep and ignorance simply by thinking of Kṛṣṇa constantly. This is called Kṛṣṇa consciousness, or samādhi.
(Bhagavad-Gita---1:24---purport).