skylight

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An overhead window, as in a roof, admitting daylight.
• noun Light from the sky.
• noun A hole in the congealed surface of a lava flow, through which flowing lava can be seen.

from The Century Dictionary.

• noun A window placed in the roof of a house, or in a ceiling; a frame set with glass, whether horizontal or in one or more inclined planes, and placed in a roof or ceiling, or in some cases, as in photographers' studios, forming a considerable part of the roof, for the purpose of lighting passages or rooms below, or for affording special facilities for lighting, as for artists' or photographers' needs.
• noun The light of the sky; specifically, the blue or pale white color of the aërial vault.
• noun Illumination by means of the light of the sky: as, a studio with a good skylight.

from the GNU version of the Collaborative International Dictionary of English.

• noun A window placed in the roof of a building, in the ceiling of a room, or in the deck of a ship, for the admission of light from above.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A window, dome, or opening in the roof or ceiling, to admit natural light.
• noun 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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a window in a roof to admit daylight