境 border

as in, "Today's ( = 今日, pronounced KYOU) border will vanish tomorrow, when The Man starts his next war." Ooh, political!!

If you stand on the earth over there, you can see all the way to the border

environment ★★★☆☆
環 (environment) + 境 (border) = 環境 (environment)

environment (like English, this is not limited to nature. A bad kid can be a "product of his environment.")

national border ★★☆☆☆
国 (country) + 境 (border) = 国境 (national border)

boundary ★★☆☆☆
境 (border) + 界 (the world) = 境界 (boundary)

means border or boundary of property (80% the same usage as さかい　（境） , but a bit more formal).

BOTH 境　and 境界　are often used in the figurative sense - like "This film pushes the envelope of good taste" or "His art blurs the boundary between painting and sculpture".

Also used in science.