Interstitial refers to the space or area between two objects or structures. It can also describe the transitional or intermediate stage between two distinct phases or states. In the medical field, interstitial is used to describe the tissue or fluid that surrounds and supports the cells and organs of the body. In advertising, interstitial refers to the ads that appear between the content on a webpage or app. Overall, interstitial can be used to describe something that exists or occurs in between two main things, whether physical or abstract.