～界
～かい · ～kai · JLPT N1 · noun
universe
🌍 界 = 田 field + 介 between: the boundary line that separates one world from another
世界 (world) and 限界 (limit) both keep this sense of a dividing line. Think of 界 as the border between two fields, so each domain is its own marked territory.
界 — field 田 + scope 介
界 combines 田 (field) as the semantic component with 介 (jiè) as the phonetic. The original sense referred to boundaries between fields, then broadened to any border, limit, or realm — yielding modern meanings like 'world' (世界) and 'boundary' (边界).