除非 (chú fēi) means 'unless' or 'only if' — it introduces the one condition that must be met for the result to happen, often paired with 否则/不然 in the second clause. It's stronger than 如果 (rú guǒ 'if'), which is a simple condition without exclusivity. Unlike 只有 (zhǐ yǒu 'only if'), which states a necessary condition with 才, 除非 emphasizes the exception that cancels a default outcome.