Pertaining to being between things, especially between things that are normally closely spaced.
The word “interstitial” comes from the Latin “interstitium” which was derived from “inter” meaning “between” + “sistere” meaning “to stand' = to stand between. 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 which involves the meshwork of lung tissue (alveolar septa) rather than the air spaces (alveoli).