Lucid
Etymology
From Latin lucidus, "bright, shining," from lucere, "to shine" (lux, "light"). The path from light to clarity is direct: what is well-lit is easy to see, and a lucid explanation lights up its subject.
adjective · /LOO-sid/
Clear and easy to understand; also, thinking clearly.
What it names
The rare gift of making a hard thing feel obvious — and, of a mind, being sharp and unclouded.
Example
Her lucid two-minute answer did what an hour of slides had failed to do.
Related idea
Perspicuous is the bookish synonym for lucid prose; a “lucid interval” means a stretch of clear-headedness.