Short for "automatically executed batch file", autoexec.bat is a startup file used with MS-DOS and early versions of Microsoft Windows operating systems (Windows 3.x and Windows 95). It contains commands that are to be executed by the operating system when the computer first boots. For example, if autoexec.bat contained the line "c:\windows\win", Windows 3.x would be executed when the computer first boots.
Windows XP, Windows Vista, Windows 7, Windows 8, and Windows 10 do not have or use the autoexec.bat file. Instead, all versions of Windows now use the registry.