It’s something that our western much-to-do culture values very little despite the meaning the word holds: an inner sense of calm and peace.
In stoic philosophy, tranquility defined as the banishment of negative emotions and the emergence of joy. It seems like the very essence of what we seek from our lives, yet it is something we very rarely claim to value.
Consider adding tranquility to your definition of success.
P.S. If this kind of thing interests you, I have some notes on stoic philosophy via my books page here.