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
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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
The walls were thicker than normal, the inner lumen almost completely obliterated.
They must be sealed to avoid moisture and kept clean to maintain lumen maintenance.
The light perceived by the human eye is measured in units called lumen-hours.
The nuclei alternate on opposite surfaces of the tubule so that the lumen remains fairly constant.
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