• 世界せかい — the world
• 限界げんかい — limit, boundary
• 業界ぎょうかい — industry, business world
• 政界せいかい — political world
• 学界がっかい — academic circles

界 combines the radical for “rice field” (田) with 介 (to intervene or mediate). It originally referred to boundaries or divisions between areas, especially fields.

Over time, it came to mean broader conceptual areas — like social spheres, professions, or the world itself.

世界せかいには たくさんの 文化ぶんかが あります。
There are many cultures in the world.

限界げんかいを 超こえる 努力どりょくが 必要ひつようです。
You need effort to go beyond your limits.

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