Viveka is the wisdom to know the eternal from the temporal.
When Buddhi is well developed, it leads to the emergence of Viveka. We can now clearly tell the difference between that which is permanent (nitya) from that which is impermanent (anitya). We clearly understand reality from the illusion of Maya. The True self of Atman stands apart from the Asmita of false identification in our mimnds. When we develop Viveka, we instinctively know the truth (Sat) from ignorance (Avidya).