interstitial (adj.)

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" (from PIE root *sta- "to stand, make or be firm"). Related: Interstices.

suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).