world, boundary
界 centers on a defined area or realm: a world, sphere, or the boundary that separates one domain from another.
界 combines 田 (field) and 介, which likely contributes the sound and the idea of dividing or separating. Together they suggest a boundary between fields, hence a defined world or realm.
A field (田) divided by a person (介) standing between them creates a boundary, defining separate worlds.
For カイ, imagine a kite flying over the boundary between two fields: kite -> カイ, and the kite marks the edge of the world.
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the world; society; the universe
business world; business circles; (the) industry
around the world; throughout the world
limit; bound
best in the world
boundary; border; limit; bounds; frontier
neighborhood; neighbourhood; vicinity
field of vision; visibility; view; visual field; (one's) ken
(world of) politics; political world; political circles
financial world; business circles
boundary
outside world; physical world
the baseball world
the earth below; the ground below
World Bank
academic world; academia; academic circles; scientific world; learned circles
legal circles
the world of sumo
bureaucracy
countries all over the world; all the countries of the world
every part of the world; all over the world; the four corners of the world
world war
worldwide; global; international; universal
worldview; outlook on the world; Weltanschauung
World Heritage (site)
the whole world
New World (esp. the Americas, but also Australasia)
the industrial world
(foreign) concession (in China); settlement
Second World War (1939-1945); World War II; WWII; WW2
economic world; financial circles; business community
nature; the natural world; realm of nature