Om Tat Sat is given in the Bhagavad Gita (chapter 17) as the threefold designation of brahman, ordained from the beginning, with which the tradition says the Vedic priests, the Vedas and the sacrifices were established. Each term carries its own weight: Om is the primordial sound and the sign of the absolute; Tat, that, points to the transcendent reality beyond name and form, the same tat as in the formula tat tvam asi; and Sat, being or the real and the good, names it as what truly is and as goodness itself.
The Gita deploys the phrase in a discussion of how acts of sacrifice, giving and austerity are to be performed, namely without attachment to their fruits and offered to the absolute, so that the formula functions as a seal placed on action to dedicate it. In later usage Om Tat Sat is spoken to consecrate rites and offerings and, in the reformist and universalist Hinduism of the modern period, has been taken up as a compact affirmation of a single reality behind the many names of God. Its brevity and scriptural authority have made it one of the most portable of Hindu formulas.