Enkrateia is often defined as self-control, but a much better definition is self-governance. The root kratia means government, and is the basis of such words as democracy (demokratia, rule by the people, plutocracy, rule of the rich, etc.).
Many modern psychologists have observed that the human personality is characterized by numerous, more or less distinct 'sub-egos': at various times we are a father, mother, child, teacher, student, worker, friend, enemy, moralist, sensualist, etc. Of great concern is to orchestrate these diverse facets into a harmonious whole, rather than a chaotic jumble of conflicting desires and actions. Effective self-government in this sense is enkrateia; its absence (a chaotic, disorganized personality), or lack of self-government, is called akrasia.
Psychologically, enkrateia can be thought of as the development or emergence a specialized sub-ego, an Inner Governor or Inner Lawgiver, which directs the others. This theme is the topic of Plato's Republic, which, contrary to popular belief, is not mainly about how to design an ideal political state; rather, it uses the example of a physical city-state as a metaphor to understand the principles of inner, self-governance.
This inner governor is sometimes in Greek philosophy called the hegemonikon (i.e., leader; related to the modern word 'hegemony'), although hegemonikon has other meanings as well.
For Plato, the only effective form this inner governor can take is that of a philosopher or lover of wisdom. Yet this philosopher leader has a most difficult task, because other sub-egos, especially those concerned with inordinate self-love and sensual pleasures, are prone to rebellion. Concerning this Plato presents an elaborate metaphor of a ship's captain and mutiny-prone crew:
[488a] ... Picture a shipmaster in height and strength surpassing all others on the ship,
The task is difficult, Plato suggests, but by no means impossible; and, in any case, necessary for happiness.
When other sub-egos succeed in wresting control of the 'ship of the self' from the philosopher leader, chaos results: the personality becomes confused, agitated, and disorganized. In a sense, this is like a kind of death. One goal of philosophy, then, is to keep the philosopher in charge, so that the genuine, authentic personality (i.e., of the harmonized kind, which functions in accord with God's will and Nature), remains in leadership continuously, i.e., a kind of 'deathless' (athanatos; thanatos = death) state.
Approaching the topic in a less mystical vein, Aristotle analyzes and offers many helpful insights about the nature of self-governance.