noun

• (architecture) A window, dome, or opening in the roof or ceiling, to admit natural light.
• (physics) 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.
• (volcanology) A hole in the upper part of a lava tube, yielding a view of the lava within.
• A region of flat, thin ice allowing a submarine to surface.