*Eudaimonia* is usually translated "happiness" but means something closer to "flourishing" or "a life going well." For Aristotle, who put the concept at the center of his ethics, eudaimonia isn't a feeling. It's the activity of a soul living in accordance with its excellences (*aretai*) — virtues — over a complete life.
This sounds abstract until you notice what it isn't. It isn't moment-to-moment pleasure (you can have a string of pleasures and still be living badly). It isn't external success (you can have wealth and fame and be miserable, or be cheated of them by bad luck and still be flourishing). It isn't a permanent psychological state (you can flourish while grieving, while struggling, while old). It's something more like: a person living rightly is doing what a flourishing human does, and the flourishing is constituted by the doing.
The Aristotelian tradition matured into modern virtue ethics (MacIntyre, Foot, Nussbaum, Hursthouse). The core insight survives: the question "what's the good life?" can't be answered by looking at any single moment or feeling. It has to be answered by what you'd recognize, watching from outside, as a life that was good to live, that was good for others, and that was good to have been. Most other ethical frameworks (deontology, consequentialism) presuppose an answer to this question and then ask how to achieve it. Virtue ethics goes after the question itself.