An opening or breach through which one can break through a problem, defense, or obstacle; a breakthrough point or opportunity.
科学者たちは新薬の研究で突破口を見つけた。
Scientists found a breakthrough in research on a new drug.
長引く交渉における突破口がようやく見えてきた。
A breakthrough in the long negotiations finally became visible.
敵陣の弱い箇所に突破口を開けた。
They created a breach at a weak point in the enemy lines.