Measurement of light output, one lumen is equal to the amount of light emitted by one candle that falls on one square foot (or meter) of surface located one foot (or meter) away from one candle.
Merriam-Webster Online Dictionary
lumen (noun)1.
the cavity of a tubular organ or part - the lumen of a blood vessel
2.
the bore of a tube (as of a hollow needle or catheter)
3.
a unit of luminous flux equal to the light emitted in a unit solid angle by a uniform point source of one candle intensity
brightness, light output, illumination, radiance, luminous flux