Interstitial: Pertaining to being between things, especially between things that are normally closely spaced. The word interstitial is much used in medicine and has specific meaning, depending on the context. For instance, interstitial cystitis is a specific type of inflammation of the bladder wall. Interstitial radiation involves placing radioactive material directly into a tumor. Interstitial pneumonia is inflammation of the lung that involves the mesh of lung tissue (alveolar septa) rather than the air spaces (alveoli).