1. Referring to a small opening or space: When Celia went to the dentist, Dr. Smith told her that she should be especially careful when cleaning the interstitial gaps between her teeth.

Sally spent the interstitial minutes between classes with rechecking her homework for the next period.
2. Pertaining to the space between tissues or cellular parts of a structure: Interstitial cystitis had developed in Josie's urinary bladder and was causing her a lot of pain!