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

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

3. Abbr. lm Physics The SI unit of luminous flux, equal to the amount of light per unit time passing through a solid angle of one steradian from a light source of one candela intensity radiating equally in all directions. See Table at measurement.