ॐ तत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः ।ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा ॥
oṃ tatsaditi nirdeśo brahmaṇastrividhaḥ smṛtaḥbrāhmaṇāstena vedāśca yajñāśca vihitāḥ purā
The formula OM, Tat, Sat, is the triple definition of the Brahman, by whom the Brahmanas, the Vedas and sacrifices were created of old.
Gītā 17·23The Gita closes this chapter with what seems at first sight a recondite utterance. The formula OM, Tat, Sat, is, it says, the triple definition of the Brahman, by whom the Brahmanas, the Vedas and sacrifices were created of old and in it resides all their significance. Tat, That, indicates the Absolute. Sat indicates the supreme and universal existence in its principle. OM is the symbol of the triple Brahman, the outward-looking, the inward or subtle and the superconscient causal Purusha.
