1. the border between two countries or regions
Usage: plural of frontier
2. the edge of settled or developed territory, especially in the American West
Usage: historical; plural of frontier
3. the limits of knowledge or achievement in a particular field
Usage: figurative; plural of frontier

Examples

• The soldiers patrolled the frontiers between the two nations.
• Pioneers pushed westward to settle new frontiers.
• Scientists are exploring the frontiers of space technology.
• The American frontiers were gradually settled during the 1800s.
• Medical researchers work at the frontiers of genetic therapy.
• These remote frontiers remained largely unexplored for decades.
• The frontiers of artificial intelligence continue to expand rapidly.