1. Of, relating to, or situated in an interstice. (wiktionary.org)
3. (Of minute animals) Living in the spaces between individual sand grains in the soil or aquatic sediments. (Google Dictionary)
Word origin: From Old French interstice (14c.) and directly from Latin interstitium “interval,” literally “space between,” from inter- “between” + stem of stare “to stand” + -al.