What is the meaning of Om Tat Sat?
The meaning of "Om Tat Sat" is given by Shri Krishna in the Bhagavad Gita. Om denotes Brahman, Tat (that) also means Brahman like in the popular mahavakya of the Upanishad Tat twam asi (you are That), and Sat (truth) also means Brahman. So these are the three names of Brahman, the absolute truth:
oṁ tat sad iti nirdeśo brahmaṇas tri-vidhaḥ smṛtaḥ
brāhmaṇās tena vedāś ca yajñāś ca vihitāḥ purā [BG - 17.23]
From the beginning of creation, the three words oṁ tat sat were used to indicate the Supreme Absolute Truth. These three symbolic representations were used by brāhmaṇas while chanting the hymns of the Vedas and during sacriﬁces for the satisfaction of the Supreme.
So in that verse Shri Krishna says "Om Tat Sat" is used during yajnas (sacrifices) and He explains on the verses following it (24,25,26,27) what the names mean.
And because reciting Gita is a yajna or sacrifice itself (BG - 18.70), at the end of each chapter "Om Tat Sat" is used just like any other yajna as explained by Shri Krishna in the above verse.
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