Adhyatma is the 24th of the 108 upanishads and is attached to the Krishna Yajur Veda. It includes this explanation of who is a jivanmukti, a person who is free from rebirth, and how to attain this supreme state.
The Unborn One is ever located in the cave of the heart. The earth is His body; though He pervades the earth, it does not know Him. The waters are His body; though He pervades the waters, they do not know Him. Agni (fire) is His body; though He pervades agni, it does not know Him. Vayu (wind) is His body; though He pervades vayu, it does not know Him. Akasha (ether) is His body; though He pervades akasha, it does not know Him. Buddhi (wisdom) is His body; though He pervades buddhi, it does not know Him. Ahamkara (ego) is His body; though He pervades ahamkara, it does not know Him. Chitta (awareness) is His body; though He pervades chitta, it does not know Him. Mrtyu (death) is His body; though He pervades mrtyu, it does not touch Him. He who is the inner soul of all creatures and the purifier of sins, He is the one divine Lord Narayana.
The wise should, through the practice of deep meditation of Brahman, relinquish the concept of "I " and "mine " in body or senses, which are not the Atma. Having known oneself as the Self, being witness to buddhi and its actions, one should ever think "So'ham " ( "I am That ") and leave behind the idea of separate Atma in others. Shunning the pursuits of the world, the body and the shastras, he should set about removing the false attribution of self. When a yogi stays always in his own Atma, his mind then perishes, having known his Atma as the Atma of all through self-experience. Never giving the slightest attention to worldly talk, think of Atma to be the Atma yourself. Then you will become Brahman and be in a blessed state.
O Sage, having dissolved jiva into all-encompassing Atma with the thought of its oneness, like the air of a jar in the universal air, be ever in a state of serenity. Having become That which is all Atmas and self-resplendent, give up both macrocosm and microcosm. Having known "I am that Brahman " in which all the universe appears like in a mirror, become one that has performed all his dharmas, O sinless one. The ever-blissful and the self-effulgent, freed from the grip of ahamkara, attains its own eternal state like the glorious moon becoming full as it always was.
With the extinction of actions arises the decay of vasanas (subconscious inclinations) and from that moksha (liberation). Who attains this is a jivanmukti. But carelessness in seeing Brahman in everything and in all places should not in the least be allowed. In a minute, maya envelops even the wise, should they become careless. Ever devoted to samadhi, become a nirvikalpi, the changeless one, O sinless! The knot of ignorance is broken completely when one sees his Atma through nirvikalpa samadhi.
Having strengthened the conception of Atma and given up the "I " in the body, one should be indifferent to any material thing. From Brahma down to a pillar, all are only unreal. Hence one should see his Atma as existing by itself. How can there be the heterogeneity in the universe made of that One Principle which is immutable, formless and all-pervasive? When there is no difference between the seer, the seen and sight, then the darkness of ignorance and the light of wisdom merge. When chitta (awareness) is not, there is nothing.
The fruit of vairagya (detachment) is spiritual wisdom, which brings then uparati (renunciation). Nivrtti, the return to One's Self, leads to the highest contentment and bliss beyond all analogy.
By separating maya from jiva (the individuated soul) and Atma, one realizes Parabrahman. Understanding this is just hearing. It becomes contemplation when such ideas are quieted. Their meaning is confirmed though concentration of the mind, called samadhi. This samadhi destroys crores of virtuous and sinful karmas which have accumulated during cycles of births.
He is a jivanmukti who preserves equanimity of mind, either when revered by the good or reviled by the vicious. He has cognized the nature of Brahman and is not subject to rebirth as before. An ascetic, having known himself, is not at all affected by any of his karmas at any time. He is Atma, all-full, beginningless, endless, immeasurable, unchangeable, replete with Truth, Consciousness and Bliss.