The most common English translation for the Koine word ἀρετή that you’re likely to come across is ‘virtue.’ This translation presents a couple of problems, which I’ll address.
Firstly, the word virtue in English has lots of baggage from its use in the Judeo-Christian tradition. Secondly, on top of that, there are certain conotations which make the word less than dynamic. Generally, when we hear virtue, even in a philosophical context, the conotation relegates the topic to moral and social applications. While it’s true that there is a moral and social component, it is not the entire story.
The next most common translation is ‘excellence,’ and this one does quite a bit better. In Diogenes Laertius 7.90, he says “Excellence (ἀρετή) is in a general sense the perfection of each thing.”
For humans, as rational critters, that means the perfection of our rational faculties.
In, The Stoic Sage by Brouwer (which I’m reading currently), the ‘dispositional definition’ of ἀρετή is discussed. The dispositional definition has to do with character, and for this case, the measure is consistency.
It’s a pretty well-known standard that excellence is a kind of knowledge (Gr: ἐπιστήμη). In the case of moral virtue, that can be cloudy. What does it mean to know a virtue? However, when viewed through the dispositional lens of ‘consistent character’ and ‘excellence’ the knowledge and praxis components of ἀρετή are more clear.