da clue's in da name....

fan·light (făn'līt')
n.
Architecture. A half-circle window, often with sash bars arranged like the ribs of a fan.
Chiefly British. A transom.

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The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2000 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.