Apatheia

Apatheia is the Stoic ideal of freedom from the pathē, the disturbing passions — fear, craving, grief, and the like — which the Stoics held to be diseases of the soul, born of false judgments about what is good and bad. The sage who sees clearly that only virtue is good is moved by none of them, and so attains a deep, unshakeable calm.

This is not coldness or indifference but the opposite: the serenity of a mind perfectly in accord with reason and nature, retaining right and rational feelings while shedding the turbulent ones. Christian ascetics later took over the word for the passionless peace of the purified soul, a kinsman of the Epicurean and Sceptic ataraxia.