f-number definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
Photog. a number that represents the relative aperture of a lens, equal to the focal length divided by the effective diameter of the lens aperture: a higher number indicates a smaller opening
f-number definition by American Heritage Dictionary
The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The ratio of the focal length of a lens or lens system to the effective diameter of its aperture. Also called f-stop.
Origin: f(ocal length) + number.