In biology, a lumen (pl.: lumina) is the inside space of a tubular structure, such as an artery or intestine. It comes from Latin lumen 'an opening'.

It can refer to:

• the interior of a vessel, such as the central space in an artery, vein or capillary through which blood flows
• the interior of the gastrointestinal tract
• the pathways of the bronchi in the lungs
• the interior of renal tubules and urinary collecting ducts
• the pathways of the female genital tract, starting with a single pathway of the vagina, splitting up in two lumina in the uterus, both of which continue through the fallopian tubes
• the fluid-filled cavity forming in the blastocyst during pre-implantation development called the blastocoel

In cell biology, lumen is a membrane-defined space that is found inside several organelles, cellular components, or structures, including thylakoid, endoplasmic reticulum, Golgi apparatus, lysosome, mitochondrion, and microtubule.