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.

Translation: "May faith grant light."

Motto of St. Gregory's College, Shawnee, Oklahoma, USA.

They shall have the light of life.

Motto of Atlantic Christian College, Wilson, North Carolina, USA.

Translation: "In Thy light we shall see the light."

Also translated as, "In Thy light shall we see light." Motto of Columbia University, New York City, USA; the College of Great Falls; Great Falls, Montana; and Ohio Wesleyan University in Delaware, Ohio.

The gradual loss of efficacy of a lamp over time, typically caused by the loss of chemical additives in the lamp and wall darkening in the arc tube: Lumen depreciation can be indicated by a curve depicting the pattern of decreasing lucid output.

The florescent illumination in Mildred's kitchen started to flicker, apparently because of the lumen depreciation in the tubes.

 

Translation: "Through the light of knowledge the people will find a way."

Motto of Texas College, Tyler, Texas, USA.