| n. | 1. | (Photom.) 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. |
| 2. | (Biol.) An opening, space, or cavity, esp. a tubular cavity; a vacuole. |
| Noun | 1. | lumen - a unit of luminous flux equal to the amount of light given out through a solid angle of 1 steradian by a point source of 1 candela intensity radiating uniformly in all directions Synonyms: lm |
| 2. | lumen - a cavity or passage in a tubular organ; "the lumen of the intestine" |