Dictionary.com
definitions

focal length

noun Optics.
  1. the distance from a focal point of a lens or mirror to the corresponding principal plane. Symbol: f
  2. the distance between an object lens and its corresponding focal plane in a telescope.

Origin of focal length

First recorded in 1745–55
Also called focal distance.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for focal distance

focal length

focal distance

noun
  1. the distance from the focal point of a lens or mirror to the reflecting surface of the mirror or the centre point of the lens
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
focal distance in Medicine

focal distance

n.
  1. focal length

focal length

n.
  1. The distance from the surface of a lens or mirror to its focal point.focal distance focus
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
focal distance in Science

focal length

[fō′kəl]
  1. The distance between the optical center of a lens or mirror to its focal point.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
focal distance in Culture

focal length

The distance between the focal point of a lens (that is, the point at which the lens will focus parallel rays of light) and the lens itself.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.