ATARAXIA and APONIA are Greek terms which are the cause of much confusion in modern understanding of Epicurean philosophy.
ATARAXIA is the Greek word for "absence of disturbance." APONIA is the Greek word for "absence of pain." These terms are often confused as the goal of life in Epicurean philosophy, but this is not the case. PLEASURE is the guide of life, with happiness (the experience of pleasures predominating over pains) being the Epicurean definition of the goal for living over time.
Given the central focus on pleasurable living as the goal, Epicurus logically concluded that in order to describe the best life possible, such a life would be described as a life in which nothing but pleasures are experienced, and in which there would be no interruption or disturbance of those ongoing experiences of pleasures.
Much modern discussion is devoted to considering APONIA and ATARAXIA as abstract concepts describing some kind of "continuing state" which some people seem to think represents states which are different from pleasure. In Epicurean philosophy, however, the fundamental concepts are that (1) there is nothing desirable in itself but pleasure, (2) all things are in motion, (3) nothing is eternal except the atoms and void, and (3) all things are constantly changing. There is therefore in Epicurean philosophy no "final state" which is representative of a single goal for all humans to seek as a final goal.
It is therefore more clear to understand APONIA and ATARAXIA in their adverbial context - meaning that best life is that of filling ones experiences with pleasure "without pain" ("aponically") and without disturbance ("ataraxically").