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A type of program error that requires that the program responsible for the error be shut down. Software applications communicate with operating systems and other applications through layers of code. An exception is an error alert that communicates the problem across the layers. When an error is detected, the exception is sent through the layers of code one by one until code is found that can handle the error. If no code has been included in any layer of the program to handle the specific error, the exception will travel through all the layers, create a fatal exception error, terminate the application and may even shut down the operating system.
For example, a program may request that the operating system reserve a block of memory for the program's use. If the operating system is unable to honor the request - if the memory request is too large or if there is not enough available memory - it will send a memory exception to the layer that made the request. The exception will continue to travel through the layers until it is dealt with. If the program does not catch the exception, because the programmer did not write code to handle that particular exception, the exception makes its way to the top layer and the operating system recognizes it as an unhandled exception and shuts down the program, resulting in a fatal exception error. Well-designed software, however, handles all exceptions.
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