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:

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.

Transluminal procedures are procedures occurring through lumina, including:^{[citation needed]}