Several parts combine into one character.

界 is a phonetic-semantic compound: 田 (field) + 介 (jiè, "between / interpose" — providing the sound). The composite reads as "the border between fields" = boundary, demarcation, edge. From this concrete agricultural image (the ridges between rice paddies that mark property and irrigation divisions) grew an extraordinary semantic spread: physical boundary → realm / domain → sphere of activity / professional world. The 界 suffix attaches productively to nouns to mean "the world of X / X-industry / X-circles."

Korean reading "gye." 世界 (segye, world — the universe of human concern), 境界 (gyeonggye, boundary), 業界 (eopgye, industry / business sector — used widely in business news), 政界 (jeonggye, political world / political circles), 限界 (hangye, limit), 視界 (sigye, field of vision / sightline). Korean uses the 界 suffix actively: 학계 hakgye (academia), 연예계 yeonye-gye (entertainment industry), 재계 jaegye (business circles).

Mandarin jiè, 4th tone. 世界 (shìjiè, world), 边界 (biānjiè, border / frontier), 界限 (jièxiàn, limit), 教育界 (jiàoyùjiè, education sector), 文化界 (wénhuàjiè, cultural circles), 体育界 (tǐyùjiè, sports world). Mandarin uses the 界 suffix as productively as Korean does.

Japanese on-reading カイ (kai) — 世界 (sekai, world), 境界 (kyōkai, boundary), 業界 (gyōkai, industry — central Japanese business term: 業界トップ "industry top"), 限界 (genkai, limit), 政界 (seikai, political world), 学界 (gakkai, academia). Japanese uses 〜界 productively: アニメ界 anime-kai (the anime world), スポーツ界 supōtsu-kai (sports world). The 〜界 suffix is one of the most semantically generative patterns in Japanese journalism and casual writing.

Memory aid: the dividing lines between fields (田) — physical boundaries became professional and conceptual realms.