A callback may refer to any of the following:
1. A security feature that verifies the dial-in user is who they say they are by calling the person back. After a callback once a username, password, and any other information has been verified they will be granted access.
2. A callback is also another name for a returned phone call.
3. In computer programming, a callback is a function, procedure, or method, which is passed as an argument to another function. The callback is then called later when the appropriate conditions are met. Callbacks are often lambda functions, and are a characteristic component of many programming paradigms including event-based programming.