The "keyup" event is a type of event in programming that occurs when a user releases a key on their keyboard. It is commonly used in web development to detect user input in forms or interactive applications. When a key is released, the event triggers a function that can perform actions based on the key that was pressed. In JavaScript, the "keyup" event can be captured using event listeners. Developers can attach these listeners to elements like text fields or the entire document. This allows for real-time feedback, such as validating input or updating the user interface based on the user's actions with the keyboard.