IPA: /ˌɪn.təˈstɪʃ.əl.i/
KK: /ɪnˈtɜrˌstɪʃ.əl.i/
In a way that occurs in the spaces between things or parts.
The medication was delivered interstitially, allowing for better absorption.
Interstitially → It is formed from "inter-" (meaning between) and "sistere" (meaning to stand). The word describes something occurring in the spaces between things.
Think of something happening 'between' ('inter-') the places where things 'stand' ('sistere') — that's why interstitially refers to occurrences in the spaces between.
No commonly confused words.