It may look like a sci-fi term, but “ataraxy” means “a state of serene calmness.” It’s based on the Greek “ἀταραξία,” meaning “impassiveness” or “lack of disturbance,” and it came into English from French in the early 1600s. Today, “ataraxy” is sometimes used as a synonym for “deep relaxation” or “serenity,” but the idea was developed by Stoic philosophers in ancient Greece, who used the term to describe a state of emotional balance that resulted from living in harmony with nature. The Stoics also thought ataraxy could be achieved by abandoning passions in favor of reason. While ataraxy might now be associated with a pleasant evening at home, Stoics encouraged soldiers entering battle to cultivate ataraxy, since mental stillness would help protect them in combat.