Final answer:
Virtue, as described by Aristotle, is the mean between a deficiency and excess. It involves doing the right thing in the right way, promoting human well-being. Virtuous actions and development are central to living a flourishing life, but additional factors like success and friendship also contribute to eudaimonia.
Step-by-step explanation:
Aristotle's Nichomachean Ethics describes virtue as promoting human well-being. To determine what actions are virtuous, Aristotle proposes that virtue is the mean between a deficiency and excess. Vices, the opposite of virtues, are deficiencies or excesses. Aristotle uses bravery as an example (Book II, Chapter 7, §2). Bravery is virtue that involves having the right amount of fear and confidence. It is the mean between excessive fear and deficient confidence on one hand (cowardice) and deficient fear and excessive confidence (rashness) on the other hand. In this way, the virtuous action will be the golden mean, neither too much nor too little. Virtue thus describes being able to do the right thing in the right way, a quality that contributes to one's well-being.
A good will is good not because of what it effects, or accomplishes, not because of its fitness to attain some intended end, but good just by its willing, i.e. in itself; and, considered by itself, is to be esteemed beyond compare much higher than anything that could ever be brought about by it in favor of some inclination. (Kant 1997a, 4:394)
Again, of all the things that come to us by nature we first acquire the potentiality and later exhibit the activity (this is plain in the case of the senses; for it was not by often seeing or often hearing that we got these senses, but on the contrary we had them before we used them, and did not come to have them by using them); but the virtues we get by first exercising them, as also happens in the case of the arts as well. For the things we have to learn before we can do them, we learn by doing them, e.g. men become builders by building and lyreplayers by playing the lyre; so too we become just by doing just acts, temperate by doing temperate acts, brave by doing brave acts.
Aristotle was careful to note in Nicomachean Ethics that virtuous development alone does not make a flourishing life, though it is central to it. In addition to virtuous development, Aristotle thought things like success, friendships, and other external goods contributed to eudaimonia.
For Plato and Aristotle, eudaimonia is related to the virtue or excellence of something (arête). Virtue or excellence is determined by the nature and purpose of something. For humans, one simply needs to determine the virtues that are proper to human nature and practice them to flourish in life. Moreover, flourishing in life gives an indication that one is acting well or virtuously. For Aristotle, virtue alone was not sufficient for flourishing. After all, someone could be very virtuous and suffer a grave misfortune. Suffering seems antithetical to flourishing. However, ancient Stoics believed that virtue was sufficient for flourishing and that tragic circumstances could not rob someone of their flourishing, because it could not take away their virtue.