add. ·noun an opening, space, or cavity, ·esp. a tubular cavity; a vacuole.
2. lumen ·add. ·noun a unit of light flux, being the flux through one square meter of surface the illumination of which is uniform and of unit brightness.
3. lumen ·add. ·noun 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.