skylight

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An overhead window, as in a roof, admitting daylight.
2. n. Light from the sky.

Century Dictionary and Cyclopedia

1. n. 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.
2. n. The light of the sky; specifically, the blue or pale white color of the aërial vault. The light of the sky is, in general, reflected sunlight. Its color is due to the size of the reflecting particles, blue light coming from the finest gaseous particles and red light from larger vaporous ones. Blue light may also come from the molecules of the gases mixed together in the atmosphere. The white light with which the blue is adulterated comes from larger particles or globules of water that reflect all the colors of the sunlight. The blue light is polarized in a plane perpendicular to the sun's rays, the white light in the plane of the sun's rays; the resultant light is polarized according to the law combining these two elementary conditions.
3. n. Illumination by means of the light of the sky: as, a studio with a good skylight.

Wiktionary

1. n. A window, dome, or opening in the roof or ceiling, to admit natural light.
2. n. 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

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. n. a window in a roof to admit daylight