noun, plural lumens, lumina [loo-muh-nuh] .
Optics. the unit of luminous flux, equal to the luminous flux emitted in a unit solid angle by a point source of one candle intensity. Abbreviation: lm
Anatomy. the canal, duct, or cavity of a tubular organ.
Botany. (of a cell) the cavity that the cell walls enclose.

1870–75; < Neo-Latin, special uses of Latin lūmen (stem lūmin-) light, window Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
lumen (ˈluːmɪn)
n , pl -mens, -mina
1.  lm the derived SI unit of luminous flux; the flux emitted in a solid angle of 1 steradian by a point source having a uniform intensity of 1 candela
2.  anatomy a passage, duct, or cavity in a tubular organ
3.  a cavity within a plant cell enclosed by the cell walls
[C19: New Latin, from Latin: light, aperture]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

unit of luminosity, 1898, coined 1894 in Fr. (A. Blondel) from L. lumen "light," related to lucere "to shine" (see light (n.)).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

lumen lu·men (lōō'mən)
n. pl. lumens or lu·mi·na (-mə-nə)

  1. The inner open space or cavity of a tubular organ, as of a blood vessel.

  2. The unit of luminous flux in the International System of Units, that is equal to the amount of light given out through a solid angle by a source of one candela intensity radiating equally in all directions.

lu'men·al or lu'min·al adj.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
lumen   (l'mən)  Pronunciation Key 
Plural lumens or lumina
  1. The central space within a tube-shaped body part or organ, such as a blood vessel or the intestine.

  2. The SI derived unit used to measure the amount of light passing through a given area per second. One lumen is equal to the luminous flux passing per unit solid angle from a light source with a strength of one candela.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Case-lighting-lumens are directly proportional to awesomeness.
Efficiency is measured by the number of lumens per watt a bulb provides.
Light output is measured in lumens at the light source.
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