Eudaimonia (also spelled eudaemonia)(/juːdɪˈmoʊniə/; Ancient Greek: εὐδαιμονία [eu̯dai̯moníaː]) is a Greek word literally translating to the state or condition of good spirit, and which is commonly translated as happiness or welfare.

In the works of Aristotle, eudaimonia was the term for the highest human good in older Greek tradition. It is the aim of practical philosophy-prudence, including ethics and political philosophy, to consider and experience what this state really is and how it can be achieved. It is thus a central concept in Aristotelian ethics and subsequent Hellenistic philosophy, along with the terms aretē (most often translated as virtue or excellence) and phronesis ('practical or ethical wisdom').

Discussion of the links between ēthikē aretē (virtue of character) and eudaimonia (happiness) is one of the central concerns of ancient ethics, and a subject of disagreement. As a result, there are many varieties of eudaimonism.

In terms of its etymology, eudaimonia is an abstract noun derived from the words eû ("good, well") and daímōn ("spirit, deity").

Semantically speaking, the word δαίμων (daímōn) derives from the same root of the Ancient Greek verb δαίομαι (daíomai, "to divide") allowing the concept of eudaimonia to be thought of as an "activity linked with dividing or dispensing, in a good way".

Definitions, a dictionary of Greek philosophical terms attributed to Plato himself but believed by modern scholars to have been written by his immediate followers in the Academy, provides the following definition of the word eudaimonia: "The good composed of all goods; an ability which suffices for living well; perfection in respect of virtue; resources sufficient for a living creature."

In his Nicomachean Ethics (§21; 1095a15–22), Aristotle says that everyone agrees that eudaimonia is the highest good for humans, but that there is substantial disagreement on what sort of life counts as doing and living well; i.e. eudaimon:

Verbally there is a very general agreement; for both the general run of men and people of superior refinement say that it is [eudaimonia], and identify living well and faring well with being happy; but with regard to what [eudaimonia] is they differ, and the many do not give the same account as the wise. For the former think it is some plain and obvious thing like pleasure, wealth or honour... [1095a17].