n.[C]

1. 国境，边境；边疆[（+between/with）]
   Everything is quiet on the frontier.

   边境上一切安静。

2. 【美】（靠近未开发地带的）边远地区[the S]
3. （探索活动的）新领域；未完全开拓的领域[P1][（+of）]
   Professor Field is devoted to the exploration of the frontiers of medicine.

   菲尔德教授致力于探索医学研究的新领域。

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.