Done without official permission from the person or organisation who has the power to give it. The word points at a rule, not just at a wish — someone in charge would have had to say yes, and did not. It attaches to actions and to people: 'unauthorized access' to a computer system, an 'unauthorized absence' from work, 'no unauthorized persons beyond this point'. You meet it constantly on signs, in contracts, in company rules, and in software error messages, which is where most students first see it. Because it belongs to the language of rules, it sounds cold and official; a friend would never use it about your behaviour. There is also a softer publishing use: an 'unauthorized biography' is a life story written without the subject's cooperation or approval — perfectly legal, just not blessed by them. British English usually spells it 'unauthorised', American English 'unauthorized'; both are correct, just pick one and stay with it.