Lucent things shine or glow with light. On a clear night, the full moon is lucent in the sky.
The literary word lucent essentially means "luminous" or "radiant," glowing with a soft, bright light. You're most likely to encounter it in a poem or novel, describing the stars or white flowers in the dusk, or someone' s beautiful pale face, but it sometimes has a "clear or lucid" meaning as well. It comes from the Latin lucentem, from lucere, "to shine."