Meaning of “unless” - Learner’s Dictionary

unless

conjunction us uk /ənˈles/
Extra Examples
I'll go first unless you'd prefer to.He holds the office for five years unless an election is called earlier.We can go out for a meal unless you'd rather stay in.I can't come to the party unless I can find a babysitter.You shouldn't try and fix it yourself unless you know exactly what you're doing.

B1 except if:

I won't call you unless there are any problems.

(Definition of “unless” from the Cambridge Learner’s Dictionary © Cambridge University Press)