Answer:
1. the extreme limit of settled land beyond which lies wilderness, especially referring to the western US before Pacific settlement. 2.the extreme limit of understanding or achievement in a particular area.
3.The border between United States and Canada is an example of a frontier. When there is a new plot of land that has never been explored, this is an example of a new frontier. When there is a new scientific theory that has not yet been explored, this is an example of a new frontier.