An interrupt is a signal from a peripheral device, like a printer or a scanner, attached to a computer; or from a program running on the computer itself that causes the computer's operating system to suspend the task it is currently working on and do something else instead. This "something else" is most often something to do with the device that sent the interrupt, e.g., service the printer, read a key from the keyboard, and so on.

The code that runs to handle the interrupt is often called an "interrupt service routine" or ISR. ISRs are typically run by the operating system, which is the main program in the computer, and which acts as host to the other programs the computer can run. Windows, MacOS, Linux, and UNIX are all operating systems. The operating system is to the computer what the laws of physics are to us...