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 waiting for data to load without blocking the main program flow. In web development, callbacks are frequently used with APIs and libraries, such as jQuery or Node.js. For example, when making a request to a server, a callback function can be defined to process the response once it arrives, ensuring that the application remains responsive during the wait.