lucid

When you call something lucid, you mean it's "easy to understand" or "sensible": "The singer's lucid style was obscured by backup singers chanting 'Shama-lama-ding-dong.'"

Lucid is ultimately from Latin lux meaning "light," and its current senses are all based on the original meaning "shining, luminous." On the one hand it refers to things that can be understood without difficulty ("a lucid definition"), on the other to sound states of mind (a madman might have "lucid intervals"). In a lucid dream, you're aware that you are dreaming and you may be able to control the dream.