(Sanskrit सत्) Literally, "existence, reality, truth, that which is, being, essence, enduring." Usually translated as "absolute existence," indicating the primordial emptiness or potentiality from which everything emerges and returns.
"Brahman is Sat, the Absolute, Reality. That which exists in the past, present and future; which has no beginning, middle and end; which is unchanging and not conditioned by time, space and causation; which exists during the waking, dream and deep sleep states; which is of the nature of one homogeneous essence, is Sat. This is found in Brahman, the Absolute. The scriptures emphatically declare: "Only Sat was prior to the evolution of this universe." —Swami Sivananda
"...OM TAT SAT, the symbol of Para Brahman or Para Brahman Itself."—Swami Sivananda
OM TAT SAT is a mantra given by Krishna in the Bhagavad-gita.