"Om Tat Sat": This has been declared to be the triple designation of Brahman. By that were created formerly, the Brahmanas, the Vedas and the sacrifices.
In simple words
Krishna teaches the sacred formula: "'Om Tat Sat' — these three words together represent the infinite reality. From the beginning, the seekers of truth, the sacred texts, and all sacred acts have been connected to these three words."
Word-by-word meanings
तत्सत्Om Tat Satइतिthusनिर्देशःdesignationब्रह्मणःof Brahmanत्रिविधःthreefoldस्मृतःhas been declaredब्राह्मणाःBrahmanasतेनby thatवेदाःVedasचandयज्ञाःsacrificesचandविहिताःcreatedपुराformerly
Contemporary scholarly and practical interpretations for modern seekers.
Swami Sivananda
17.23 तत्सत् Om Tat Sat? इति thus? निर्देशः designation? ब्रह्मणः of Brahman? त्रिविधः threefold? स्मृतः has been declared? ब्राह्मणाः Brahmanas? तेन by that? वेदाः Vedas? च and? यज्ञाः sacrifices? च and? विहिताः created? पुरा formerly.Commentary Om Tat Sat is the root of the entire universe. Om is the Akshara Brahman. Tat means Thath? the indefinable. Sat means Reality.Para Brahman? that Supreme Being? the abiding place of all that lives and moves? is beyond name and class. The Vedas have ventured to give a name to Him. A new born child has no name but no,receiving one he will answer to it. Men who are troubled by the afflictions of this world run to the Deity for refuge and call Him by the name. When Brahman is invoked through the name that which is hidden is revealed to the aspirant.These three words have a divine power of their own. The vibrations that they produce in one are such as to arouse the latent divinity and also to secure the necessary response from the Cosmic Being Whom they connote.When a sacrificial rite or the like is found defective? it will be rendered perfect by the utterance of the powerful Mantra Om Tat Sat or one of the three designations in the end. With Om or Om Tat Sat all acts of sacrifice? study of sacred scriptures? spiritual discipline and meditation are commenced. If the doer of sacrifices remembers either of these Mantras all obstacles that stand in the way of success of the sacrifices are removed.Om Tat Sat has been declared to be the triple designation of Brahman in the Vedanta by the knowers of Brahman. The power of creation that lies in the Creator emanates from this Mantra. When He meditated inwardly on the meaning of this Mantra and repeated the threefold word? He acired the power to create. Then He created the Brahmanas? gave them the Vedas to be their guide and directed them to perform sacrifices and other rites.Puraa Of old At the beginning of creation by the Prajapati.Brahman here means the Veda.
Swami Chinmayananda
This interpretation draws on a specific tradition and may not represent the view of any single school. For authoritative guidance within a specific tradition, seek a qualified teacher.
# BG 17.23 — The Meaning
The word "tat" (that) is a designation by which any object is indicated. In this manner, "tat sat" is understood as the threefold designation of Brahman — that is, each of these words points to Brahman alone. In the ordinances of ritual action, when performing duties, one is taught to remember and recite such designations, through which any incompleteness or defect remaining in the performance of action is removed.
Every action yields its fruit, yet that fruit depends not merely upon the action itself, but also upon the purity of intention of the one who acts. However diligently a person may perform any religious observance, if their intention is of a lower order, that observance remains powerless to bestow the highest fruit upon the performer. Though our actions may appear similar, the fruit received by one person differs from that received by another. The cause of this can only be the quality of the actor's intention.
Through remembrance of the Divine, we may render our intentions ever more luminous and radiant. Only by releasing our identification with the non-self coverings can we abide in our supreme nature. To the extent that our actions are selfless, to that extent the reward obtained shall be pure. For the aspirant to destroy egoism, it is necessary to possess awareness of one's spiritual essence.
That essence is symbolized by what is unborn, imperishable, beyond all coverings, and the substratum of all bodily conditions. The word "tat" points to the Supreme Brahman. In the celebrated mahavakya of the Upanishads, "tat tvam asi," the word "tat" indicates that supreme truth which is the source of the origin, sustenance, and dissolution of the entire cosmos — the Brahman that is the cause of the universe is indicated by the word "tat."
"Sat" means existence unobstructed by time — past, present, or future. This sattvic nature pervades everywhere. Thus, through these three words — "tat sat" — to remember the Supreme Self, transcendent of the universe, the cause of the universe, and pervading the universe, is to establish unity with that Divine.
Through remembrance of the Divine, our actions become purified. From Brahman, as designated by "tat sat," have arisen all the classes of humanity, all dharma, all the Vedas, and all sacrifice. The substratum is the cause of all manifest creation. Now, in the verses that follow, the ordinance for the use of these three words shall be explained.