lumen Definition

lumen
noun lumina, lumens

1. physics.
(symbol lm) in the SI system: a unit of luminous flux, defined as the amount of light emitted by a point source of intensity one candela within a solid angle of one steradian.
2. biol.
In living organisms: the space enclosed by the walls of a vessel or tubular organ, eg within a blood vessel or intestine.

Etymology: 19c: Latin, meaning ‘light'.