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.
he lumen (symbolized lm) is the International Unit of luminous flux. It is defined in terms of candela steradians (cd multiplied by sr). One lumen is the amount of light emitted in a solid angle of 1 sr, from a source that radiates to an equal extent in all directions, and whose intensity is 1 cd.
Lumens are related to lux in that one lux is one lumen per square meter. A full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 cd × 4π sr = 4π cd⋅sr ≈12.57 lumens.