A lumen (Lat. lūmen, an opening or light) (pl. lumina) is an inner space, lining or cavity.
- The interior of a vessel within the body, such as the small central space in an artery or vein, or any of their relating vessels through which blood flows.
On a larger scale it may refer to
- The interior of the gastrointestinal tract 
- The pathways of the bronchi in the lungs
- The interior of urinary collecting ducts
- The single pathway of the vagina