“boundary / border” in Japanese is 境, read kyou, kei, sakai (きょう, けい, さかい). It is a Grade 5 Kyōiku kanji, taught in year 5 of Japanese elementary school, and appears in words such as 国境 (national border).
What does 境 mean?
国の境を越える。
Same sound, different character — the classic trap. Compare them side by side: