Enkrateia (Greek ἐγκράτεια, "in power (over oneself)") from ἐν (en, “in”) + κράτος (krátos, “power”). Enkrateia comes from the adjective enkratês (ἐγκρατής from ἐν (en, “in”) + κράτος (krátos, “power”)) meaning possession, power over something or someone else.
During Socrates' life three of his disciples, Isocrates, Xenophon and Plato, transformed the adjective enkratês to the noun enkrateia and gave it a different meaning: with them, enkrateia meant not power over something or somebody else but power over yourself, power over your own passions and instincts, self-control.
For Aristotle, enkrateia is the antonym of akrasia (ἀκρασία from ἀ = without + κράτος = power, control) which means "lacking command (over oneself)". In this sense, enkrateia is the state of performing what is known to be a positive choice because of its positive consequences as opposed to akrasia, which is the state of performing what is known to be not a positive choice (because of its negative consequences), but nevertheless performing it because of its immediate pleasures.
To Xenophon, enkrateia is not a particular virtue but "the foundation of all virtues".