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border; boundary
turning point; watershed
area; region; spot; space; environment
psychological state; mental state
cognitive object; something perceptible by the sense organs or mind
この川が二つの県の界になっている。
This river forms the boundary between the two prefectures.
界を越えて向こう側に行った。
I crossed the border and went to the other side.
昔はこの石が村の界だったらしい。
Apparently this stone used to mark the village boundary.