The classical inquiry
Sōphrosynē — translated variously as "self-control," "moderation," "temperance," and "soundness of mind" — is one of the four virtues that organise the classical Greek moral vocabulary. The word is older than philosophy: Homer already uses it, and it carries from the start the sense of a person being of sound and ordered mind rather than carried about by appetite or impulse.
Plato's Charmides is the dialogue devoted to the inquiry into sōphrosynē, and like the Laches it ends in aporia. In the Republic the virtue takes a structural role: it is what holds the parts of the well-ordered soul (and the well-ordered city) in their proper relation, an agreement between rulers and ruled about who should rule. The Aristotelian treatment in the Nicomachean Ethics gives temperance as a virtue of character governing the pleasures of touch and taste, lying in a mean between licentiousness and insensitivity.
The Xenophontic Socrates of the Memorabilia is, more than any other ancient portrait, the figure of practical enkrateia (self-mastery): he is shown again and again governing appetite, fatigue, fear and anger as a precondition of giving or receiving good counsel.
What the tradition adds
The Stoics make enkrateia and the related virtues central; the Christian tradition receives temperance as the fourth cardinal virtue and treats it together with the Pauline emphasis on the governance of the body. The early-modern moralists (Montaigne, who reads the classical texts seriously, the Roman moralists they draw on) all return to it.
Why it matters for Virtue & Power
Self-control sits at the centre of the platform's treatment of leadership. The classical conviction that no one is fit to govern others who cannot govern himself is one of the points the texts return to most often, and it is one of the clearest cases of a classical thought that is not so much "still relevant" as never having gone away.