Definition of "lumen"

(physics) In the International System of Units, the derived unit of luminous flux; the light that is emitted in a solid angle of one steradian from a source of one candela. Symbol: lm.  examples

(anatomy) The cavity or channel within a tube or tubular organ.  examples

(botany) The cavity bounded by a plant cell wall.  examples

(medicine) The bore of a tube such as a hollow needle or catheter.  examples