1 border between countries
ADJ. common Neither country would guarantee the integrity of their common frontier. | eastern, northern, etc. There were very few border controls on the south-western frontier. | Franco-Spanish, Hispano-French, Spanish-French, French, etc.
VERB + FRONTIER cross The army crossed the frontier in the middle of the night. | control, defend, guard The rebels control the frontier and the surrounding area.
FRONTIER + NOUN controls, post | guard | area, province, region, town, zone | line
PREP. across the ~ Many people travelling across the frontier were illegal immigrants. | along the ~ an army grouping along the frontier | at the ~ There was an army checkpoint at the frontier. | on the ~ people living on the German frontier | over the ~ They were forced to retreat back over the frontier. | ~ between the frontier between India and Pakistan | ~ with France's frontier with Germany
2 border between what we know and do not know
ADJ. final, new
VERB + FRONTIER explore Space is the final frontier for us to explore. | advance, push back The scientists' work will push back the frontiers of physics.