frontier

noun

1. an undeveloped field of study
a topic inviting research and development

e.g. he worked at the frontier of brain science

2. an international boundary or the area (often fortified) immediately inside the boundary

3. a wilderness at the edge of a settled area of a country

e.g. the individualism of the frontier in Andrew Jackson's day

1. 边界;边境
A frontier is a border between two countries.

e.g. It wasn't difficult then to cross the frontier.
那时穿越边境并不困难。

2. 西部边疆,边远地区(指 20 世纪前美国西部欧洲移民尚未定居的部分)
When you are talking about the western part of America before the twentieth century, you use frontier to refer to the area beyond the part settled by Europeans.

e.g. ...a far-flung outpost on the frontier.
远驻西部边疆的前哨

3. (尤指知识的)前沿，尖端，新领域
The frontiers of something, especially knowledge, are the limits to which it extends.

e.g. ...pushing back the frontiers of science.
推进科学前沿
e.g. ...technological frontiers.
技术尖端