Source : Webster's Revised Unabridged Dictionary (1913)

Lumen \Lu"men\, n.; pl. L. {Lumina}, E. {Lumens}. [L., light, an
   opening for light.]
   1. (Photom.)
      (a) A unit of illumination, being the amount of
          illumination of a unit area of spherical surface, due
          to a light of unit intensity placed at the center of
          the sphere.
      (b) A unit of light flux, being the flux through one
          square meter of surface the illumination of which is
          uniform and of unit brightness.

   2. (Biol.) An opening, space, or cavity, esp. a tubular
      cavity; a vacuole.