A "callback" is a programming concept where a function is passed as an argument to another function. This allows the first function to be executed at a later time, often after a specific event or condition is met. Callbacks are commonly used in asynchronous programming, enabling tasks like handling user interactions or processing data without blocking the main program flow. In web development, callbacks are frequently used with APIs and libraries, such as JavaScript and Node.js. For example, when fetching data from a server, a callback function can be defined to process the data once it is successfully retrieved, ensuring a smooth user experience.