skylight

noun

1. A window, dome, or opening in the roof or ceiling, to admit natural light.
2. Diffuse sky radiation—solar radiation reaching the earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere.
3. A hole in the upper part of a lava tube, yielding a view of the lava within.
4. A region of flat, thin ice allowing a submarine to surface.