Definition of unless conjunction from the Oxford Advanced American Dictionary

      

    unless

     conjunction
    conjunction
    NAmE//ənˈlɛs//
     
     
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  1. 1used to say that something can only happen or be true in a particular situation You won't get paid for time off unless you have a doctor's note. I won't tell them — not unless you say I can. Unless I'm mistaken, she was back at work yesterday. He hasn't got any hobbies — unless you call watching TV a hobby.
  2. 2used to give the only situation in which something will not happen or be true I sleep with the window open unless it's really cold. Unless something unexpected happens, I'll see you tomorrow. Have a cup of tea — unless you'd prefer a cold drink? Unless is used to talk about a situation that could happen, or something that could be true, in the future. If you know that something has not happened or that something is not true, use if… not:If you weren't always in such a hurry (= but you are), your work would be much better.Your work would be much better unless you were always in such a hurry.
See the Oxford Advanced Learner's Dictionary entry: unless