Sat is the ancient concept, found in the vedas.

Meaning of Sat-

The word ‘Sat’ means, the existence or truth.

According to the vedas and upanishads –

Sat is the one who is the true embodiment of existence among all existences.

Sat (existence) is the state of being whether of a substratum, an attribute or an action.” – Vaisheshik 1.2.7

According to the vedic philosophy – 

Sat can be affirmed of everything that exists.