lumen

The inner open space or cavity of a tubular organ, as of a blood vessel or an intestine.

The interior of a membrane-bound compartment or organelle in a cell.

The SI unit of luminous flux, equal to the amount of light passing through a solid angle per unit time from a light source of one candela intensity radiating equally in all directions.

An opening or passageway, as, in anatomy, of a hollow tubular organ.

In botany, the internal cavity, or space within the wall, of a cell.