1) Cause for a reboot, often 2) Serious software problem 
An error which makes a program stop working.
In computing, a fatal error or fatal exception error is an error that causes a program to abort and may therefore return the user to the operating system. When this happens, data that the program was processing may be lost. A fatal error is usually distinguished from a fatal system error. (colloquially referred to by the error message it produces ....