What is fatal error in autocad?

Thanks Mr. Yaqoob & Elke for defining the Fatal Error definition. I agreed with Both of You Answers.

Fatal errors usually indicate program corruption or a corruption somewhere within the operating system.

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FATAL ERROR MEANS CORRUPTION PROGRAM OR ERROR IN OS.

IT OCCURS WHEN YOU START THE AUTOCAD

When you tried to launch AutoCAD, the following error message was displayed and the product would not run. FATAL ERROR: Unhandled Access Violation Reading0x0000 Exception at4048b9h

Solution:

This error message indicates that the program installation has become corrupted. To correct the issue, the software needs to be reinstalled. This error occurs when certain files become locked during the uninstall/reinstall process. To reinstall AutoCAD correctly:

1. On the Windows Start menu, enter acsignopt in the Search programs and files field and press Enter.
2. Clear the Validate Digital Signatures and Display Special Icons option. 
3. Completely remove Autodesk software and reinstall as described in the article, Remove All Products from a Windows System.

There is a patch that Autocad put out that supposedly fixes this problem. Check their website. BUT, I have had the problem repeat itself. What I have found to work is that you reboot your computer. Too much in your memory. I like to open3 or4 drawings at the same time. Usually the computer can handle it. But not all the time. Did you load AutoCad onto another machine? Autocad is very strict on how many machines you can load their software on. If you are the licensed user, then contact Autcad. They MIGHT help you in getting the software to load. I think this error code is the response when the Software Key is corrupted or missing.