界

It is a phono-semantic compound composed of:

田 (밭 전, “field”) — indicating the meaning, related to land or borders;

介 (낄 개) — providing the sound.

결계(結界) - magical/spiritual barrier, forcefield;

경계(境界) - boundary, border

마계(魔界) - demon realm, infernal world

세계(世界) - the world

신세계(新世界) - new world

업계(業界) - industry, professional field

외계(外界) - outer world, outer space

외계인(外界人) - alien (extraterrestrial being)

천계(天界) - heavenly realm, celestial world

학계(學界) - academic world, academia

한계(限界) - limit, boundary, constraint

An alternative form (이체자) is 堺 (U+583A), which includes the 土 radical on the left and is sometimes used in proper nouns or historical texts.