Oṃ Tat Sat (ॐ तत् सत्)
BG 17.23: “Oṃ Tat Sat — this is remembered as the threefold designation of Brahman; by it the brāhmaṇas, the Vedas, and the sacrifices were ordained of old.” Chapter 17 has just sorted worship, food, austerity, and giving into sattvic, rajasic, and tamasic grades; the closing formula is what lifts an act out of that calculus of quality and offers it up.
How the three are used
BG 17.24–BG 17.27 assign each syllable a role. Oṃ begins every act of sacrifice, gift, and austerity performed by those who know Brahman. Tat (“That”) is uttered by those seeking liberation when they perform such acts without aiming at the fruit — the word points the deed away from oneself. Sat (“the Real, the Good”) names both existence and goodness: it is applied to steadfastness in sacrifice, austerity, and gift, and to “action for the sake of That.” The chapter’s last word, BG 17.28, is the counterweight: whatever is offered or done “without faith” is asat — “nothing, here or hereafter.”
Related
- Brahman (ब्रह्मन्) — the three syllables are names for Brahman; the formula consecrates by invoking it
- Yajña (यज्ञ) — Oṃ Tat Sat is spoken over acts of yajña; “Tat” in particular encodes yajña’s defining move, doing without a claim on the outcome
- Śraddhā (श्रद्धा) — chapter 17’s frame: it is faith that makes an act “Sat” rather than “Asat,” whatever its guṇa
Verses that reference this
"Aum Tat Sat," this has been considered to be the threefold designation of the Eternal. By that were ordained of old Brâhmanas, Vedas and sacrifices.
Therefore with the pronunciation of "Aum" the acts of sacrifice, gift and austerity as laid down in the ordinances are always commenced by the knowers of the Eternal.
"Sat" is used in the sense of reality and goodness; likewise, O Pârtha, the word "Sat" is used in the sense of a good work.
Steadfastness in sacrifice, austerity and gift is also called "Sat", and action for the sake of the Supreme is also named "Sat."