hidden / inward / esoteric
Active participle of بَطَنَ (to be hidden/inner). As divine name: الْبَاطِنُ (The Hidden). Noun: بَطْنٌ (interior/belly). Paired with ظَاهِرٌ.
The ẓāhir/bāṭin pair also applies to Quran interpretation - the apparent meaning and the deeper, hidden meaning.