Ataraxia (ἀταραξία, alpha privative negation of tarachê -- disturbance, trouble; hence, "unperturbedness", generally translated as "imperturbability", "equanimity", or "tranquility") is a Greek term first used in Ancient Greek philosophy by Pyrrho and subsequently Epicurus and the Stoics for a lucid state of robust equanimity characterized by ongoing freedom from distress and worry. In non-philosophical usage, the term was used to describe the ideal mental state for soldiers entering battle.
Achieving ataraxia is a common goal for Pyrrhonism, Epicureanism, and Stoicism, but the role and value of ataraxia within each philosophy varies in accordance with their philosophical theories. The mental disturbances that prevent one from achieving ataraxia vary among the philosophies, and each philosophy has a different understanding as to how to achieve ataraxia.
Being both a mental and katastematic pleasure, ataraxia has a supreme importance in Epicurean ethics and is key to a person's happiness. In the Epicurean view, a person experiences the highest form of happiness should they ever be both in a state of aponia and ataraxia at the time.
Unlike in Pyrrhonism and Epicureanism, in Stoicism ataraxia is not the ultimate goal of life. Instead, a life of virtue according to nature is the goal of life. However, according to the Stoics, living virtuously in accordance with nature would lead to ataraxia as a byproduct.
An important distinction to be made is the difference in Stoicism between ataraxia and the Stoic idea of apatheia. While closely related to ataraxia, the state of apatheia was the absence of unhealthy passions; a state attained by the ideal Stoic sage. This is not the same as ataraxia. Apatheia describes freedom from the disturbance of emotions, not tranquility of the mind. However, apatheia is integral for a Stoic sage to reach the stage of ataraxia. Since the Stoic sage does not care about matters outside of himself and is not susceptible to emotion because of his state of apatheia, the Stoic sage would be unable to be disturbed by anything at all, meaning that he was in a stage of mental tranquility and thus was in the state of ataraxia.