What is the primary characteristic of a moon?

The primary characteristic of a moon is that it is a natural satellite that revolves around a planet. This definition captures the essence of what distinguishes moons from other celestial bodies. Unlike stars that generate their own light and heat, or planets that orbit stars, moons are specifically defined by their relationship with a planet, being bound by the planet's gravitational pull and exhibiting a consistent orbital pattern around it.

Other options describe different types of celestial bodies or phenomena; for instance, illuminating the night sky is a feature of various celestial objects, including stars and reflected light from planets, but does not uniquely define a moon. Similarly, stars and planets have their own distinct characteristics separate from moons. Thus, the correct answer emphasizes the specific role of a moon as a natural satellite, making it the most accurate choice.