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