Boundary
noun
The kanji and the word are exactly the same. That means they share meanings as well.
さかい
Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you to remember: You walk up to the boundary. You're there to pick something up to take it across. This thing you're smuggling across the boundary is a sockeye (さかい) salmon. You take it across the boundary and sell it for a big profit.
コウイチは一週間も生死の境をさまよった。
Koichi hovered on the boundary between life and death for a week.
この川がちょうど両州の境となっているんですよ。
This river forms the boundary between the two states.
今日を境に三日坊主の自分に決別するんだ！
As of today, I am going to say bye to the me that never sticks with things!