ataraxia \at-uh-RAK-see-uh, noun:

A state of freedom from emotional disturbance and anxiety; tranquility.

The former breathes only peace and liberty; he desires only to live and be free from labor; even the ataraxia of the Stoic falls far short of his profound indifference to every other object.
— Jean-Jacques Rousseau, A Discourse on the Origin of Inequality and a Discourse on Political Economy

Thus, hedonism ends in ataraxia, which confirms the paradoxical relation between sadism and stoicism.
— Marquis de Sade, 120 Days of Sodom and Other Writings

Ataraxia stems from the Greek word of the same spelling that meant “impassiveness.”