diastem

例句：

1. 1. The river is a natural boundary between the two countries.
   这条河是两国的自然分界线。
2. 2.The scientist used a microscope to identify the boundaries of the new species.
   科学家使用显微镜来识别新物种的界限。
3. 3. The team decided to draw a diastem for safety reasons.
   由于安全原因，团队决定划定一个分界线。