1 something that makes a situation hard to understand or to deal with 使情况复杂的因素: Unless there are any unexpected complications, I'll be arriving next month. 除非有什么意外变化，不然我将于下月到达。

2 a new illness that you get when you are already ill 并发症: Unless he develops complications, he'll be out of hospital in a week. 如果不出现并发症，他将在一个星期内出院。