For Aristotle, the various excellences of character (courage, moderation, justice, and the like) are all instances of tending to find a balance between overdoing and underdoing a particular kind of action or feeling, either generally or in a particular respect (e.g., becoming angry in the wrong situations, toward the wrong kinds of people, too quickly, too strongly, etc.). Borrowing terminology from archery, he speaks of balance as a matter of hitting the target (σκοπός) and of imbalance as a matter of missing the mark (ἁμαρτία).
Because an excellence of character is not a single act but a settled practice or way of being (at times he likens it to a second nature acquired through long experience), it involves stable patterns of intention (προαίρεσις) and thinking things through (διάνοια). It is not only a way of conducting yourself (πράττειν) but also a way of being minded (φρόνειν), in which success involves both acting well (εὐπραξία) and deliberating well (εὐβουλία). This excellent way of being minded is φρόνησις or wisdom (often rendered as "practical wisdom" or, in older translations, as "prudence").
Interestingly, φρόνησις itself is also a balance: in Book II.3 of the Eudemian Ethics Aristotle contrasts it with cunning (πανουργία) and foolishness (εὐήθεια). Whereas cunning might involve rationalization of your behavioral waywardness (we could call this deliberating badly or δυσβουλία) and foolishness might involve blindly being led by your emotions and cravings (we could call this not deliberating at all or ὐβουλία), φρόνησις involves straight reasoning (ὀρθός λόγος) in which you correct your bad tendencies toward craving (ἐπιθυμία) for too much pleasure (τό ἡδύ) or wanting (θυμός) to gain too much advantage (τό συμφέρων); instead, you come to see that by doing and choosing what is beautifully right (τό καλόν) you experience a higher kind of enjoyment and advantage. Thus wisdom is a master virtue that guides all the particular excellences of character (more about those in my next research report and, of course, in my forthcoming book Complete Yourself: Aristotle on Personal Excellence).
UPDATE 2018-02-16
While thinking further about φρόνησις, a perplexity occurred to me. Since φρόνησις is an ἀρετή and thus itself a balance or mean between cunning (πανουργία) and foolishness (εὐήθεια), you need a way to achieve that balance. With the excellences of character, φρόνησις is the way to achieve balance. With the excellence of thought that is φρόνησις, it would seem that you need φρόνησις in order to achieve φρόνησις. Can φρόνησις guide or produce itself? How do you acquire the balance that is wisdom if wisdom is required to acquire balance? An ἀπορία indeed...
FOR FURTHER EXPLORATION