The term "unauthorized use" refers to any action or behavior that is not approved, sanctioned or permitted by an individual, organization or authority. In other words, it means using something without permission or going against the rules and regulations set for its usage. This phrase can be used in various contexts such as unauthorized access to a computer system, misuse of funds, trespassing on private property, etc. Essentially, any action that is taken without proper consent or authorization would fall under this category.