1. The space in the interior of a hollow tubular structure (artery or intestine). 2. The unit of luminous flux; the luminous flux emitted in a unit solid angle of 1 steradian by a uniform point source of light having a luminous intensity of 1 candela. 3. The volume enclosed within the membranes of a mitochondrion or of the endoplasmic reticulum. 4. The bore of a catheter or hollow needle.