The oldest definition
Before strength, wealth, or charm, nearly every culture measured a man by whether his word predicted his behavior. Roman fides, Japanese makoto, Igbo ofo — different words, one standard: the gap between what you said and what you did.
Why it compounds
Aristotle's insight is mechanical: virtue is a habit. Each kept promise makes the next one cheaper; each broken one makes the next one more expensive. Character is an interest rate, not a personality trait.
The practice
Make promises small enough that you cannot fail them, then never fail them. Expand slowly. A man who keeps ten tiny promises for a month has changed more than one who declares a transformation.