Used to state an exception to a rule; except if a certain condition is true.
“Late fees apply unless the payment is postmarked before the due date.”
“Guests are not permitted in the pool area unless accompanied by a member.”
'Unless' flips the meaning of a rule, making it a high-risk word for test-takers who read too quickly. Part 2 diagrams often use it to mark an exception to a general policy, such as 'parking is not allowed unless you have a permit,' and questions often test whether you noticed the exception.
Synonyms: except if, if not, other than when