The frontiers of something, especially knowledge, are the limits to which it extends. (尤指知识的) 前沿
noun [具体名词] 边境，国界；(常sing.) 西部边疆，边远地区；（尤指知识的）前沿，新领域；西部边疆，边远地区
It wasn't difficult then to cross the frontier. 那时候穿越国界并不难。
adjective [原级] 边境的，边疆的
The rebels control the frontier and the surrounding area. 叛乱分子控制了边疆地区。
- frontier 出现在下面的课文中（提供视频、音频及笔记等可系统性学习）
- Lesson21 Williams S. Hart and the early 'Western' film
