Boundary
noun
The boundary separates worlds. It is a boundary.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
あの橋があるところが、二つの国の境界です。
The place where that bridge is located is at the border between two countries.
最近、妻に心の境界線を引かれている気がする。
Lately, I have the sense that my wife has established emotional boundaries between us.
母と私は、いつもお互いの個人的な境界線のようなものを気にしています。
My mom and I are always cautious of each other’s personal boundaries.