Ataraxia
Etymology
From Greek "a-" (without) and "tarachē" (disturbance or trouble), from "tarassein" (to disturb), giving "freedom from disturbance." It was a shared goal of several ancient schools — the Epicureans, the Stoics, and especially the Pyrrhonist skeptics, who sought it by suspending judgment. It entered English as a technical term of philosophy.
noun · /at-uh-RAK-see-uh/
A state of serene calm and freedom from mental disturbance.
What it names
A precise philosophical term for tranquility reached by releasing anxious striving and judgment — useful for the specific peace that comes from ceasing to be troubled rather than from getting what you want.
Example
After weeks of meditation he approached something like ataraxia: the news still came, but it no longer knocked him off balance.
Related idea
Close to equanimity (already here), but ataraxia carries the specific philosophical program of reaching calm by letting go of anxious judgments, not merely staying composed under pressure.