A good word definition should be characterized by its lucidity, or clarity. In other words, it should be clear and easy to understand — like this one.
Lucidity comes from the Latin lucidus meaning "light, bright, clear." Certainly when something is light, bright, and clear, it's ripe with the quality of lucidity. An easy-to-read textbook is characterized by lucidity. A person too can possess a quality of lucidity, one that would allow her to express ideas as clearly as possible.