Definitions for "Lumen"
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.
A unit of light flux, being the flux through one square meter of surface the illumination of which is uniform and of unit brightness.
A unit measurement of the rate at which a lamp produces light. A lamp's lumen output rating expresses the total amount of light the lamp emits in all directions per unit time.
An opening, space, or cavity, esp. a tubular cavity; a vacuole.
Latin = opening, hence the space within a tube.
Cavity of hollow, tubular organ, such as the intestine or blood vessels.
A hole in a tube. After a tracheostomy, there is a lumen in the front of the tracheal wall.
Keywords:  hole, artery, flows, blood
The hole in the artery through which blood flows.