1. In physics, the SI unit (International System of Units) of luminous flux, equal to the amount of brightness crossing a unit area at a distance from a light source of intensity of one candela (symbol lm): The lumen is based on the spectral sensitivity of the photosensors in the human eyes under high (daytime) radiant levels.
2. In anatomy, the space inside any tubular structure in the body: Some of the body tubes, which provide lumen, include the intestines, arteries, and veins.

The surgeon, Dr. Smith, could determine whether there was a blockage in the lumen of his patient's intestine.

3. In botany, the cavity within a plant cell wall: The assignment to the students by the botany instructor, Mr. Jones, was to make detailed drawings of the lumen of three different plants, in order to compare their cellular construction.