Bhagavad Gita · Chapter 17: The Three Kinds of Faith
Bhagavad Gita 17.23
ॐ तत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः ।
ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा ॥
oṃ tatsaditi nirdeśo brahmaṇastrividhaḥ smṛtaḥ | brāhmaṇāstena vedāśca yajñāśca vihitāḥ purā ||
"Om Tat Sat": this is held to be the threefold name of Brahman. By it the brahmins, the Vedas, and the sacrifices were ordained in ancient times.
Modern context
'Om Tat Sat' is the threefold name of the absolute. Om is the sacred sound; Tat means 'That', pointing beyond all names; Sat means 'what is real and good'. Together they form a kind of seal that can be placed on any act, dedicating it to the highest. Many people still write or speak these words at the start or end of important work.