n.[C]

1. 國境，邊境；邊疆[（+between/with）]
   Everything is quiet on the frontier.

   邊境上一切安靜。

2. 【美】（靠近未開發地帶的）邊遠地區[the S]
3. （探索活動的）新領域；未完全開拓的領域[P1][（+of）]
   Professor Field is devoted to the exploration of the frontiers of medicine.

   菲爾德教授致力於探索醫學研究的新領域。

n.

1. a border separating two countries.
2. the extreme limit of settled land beyond which lies wilderness.
3. the extreme limit of understanding or achievement in a particular area.