The Collaborative International Dictionary of English v.0.48:

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.
          [Webster 1913 Suppl.]

   2. (Biol.) An opening, space, or cavity, esp. a tubular
      cavity; a vacuole.
      [Webster 1913 Suppl.]