Is it ensure, insure, or assure?
early 15c., from Old French interstice (14c.) and directly from Latin interstitium "interval," literally "space between," from inter- "between" (see inter-) + stem of stare "to stand" (see stet). Related: Interstices.
interstice in·ter·stice (ĭn-tûr'stĭs)n. pl. in·ter·stic·es (-stĭ-sēz', -sĭz) A small area, space, or hole in the substance of an organ or tissue.