φρόνησις is "practical wisdom," prudence: for the Stoics the first of the four cardinal TERMvirtues (φρόνησις, σωφροσύνη, ἀνδρεία, TERMδικαιοσύνη) and in a sense their root. It is defined as ἐπιστήμη ὧν ποιητέον καὶ οὐ ποιητέον — "the knowledge of what is to be done and not done," or the knowledge of TERMgoods, evils, and things indifferent (ἐπιστήμη ἀγαθῶν καὶ κακῶν καὶ οὐδετέρων).

The other virtues are the same φρόνησις applied to its domains: in choice, σωφροσύνη (temperance); in distribution, δικαιοσύνη (justice); in endurance, ἀνδρεία (courage) — the Stoic thesis of the mutual entailment and unity of the virtues. Since for the Stoics virtue is "knowledge" (ἐπιστήμη), φρόνησις is the healthy state of the TERMruling part: reason judging rightly. Hence its "smooth flow" (εὔρους): a mind that works unerringly moves without stumbling.

SVF III 262 ff. (the doctrine of the virtues; φρόνησις as ἐπιστήμη); DL VII (the four cardinal virtues and their definitions); Stob. Ecl. II; Cic. De fin. III; LS 61. The Aristotelian background: NE VI (the distinction of φρόνησις from σοφία, practical from theoretical wisdom), which the Stoics in effect collapse. In Marcus, φρόνησις and the cognate φρόνιμος — e.g. Med. 05-09 ("what is more agreeable than wisdom itself?"); 05-12 (φρόνησις in the list of the four cardinal virtues — the "true goods"); 05-18 (the shame if ignorance and conceit are stronger than wisdom).

In 05-09 φρόνησις is the climax of the "revaluation of the agreeable": "what is more agreeable than wisdom (φρόνησις) itself?" — when you consider the unerring and smooth-flowing working of the faculty of comprehension and knowledge. Not an asceticism against pleasure, but the higher pleasure: the εὔροια of the mind itself.