EXIT

Quit the current batch script, quit the current subroutine or quit the command processor (CMD.EXE) optionally setting an errorlevel code.

Syntax
      EXIT [/B] [exitCode]

Key
    /B        When used in a batch script, this option will exit 
              only the script (or subroutine) but not CMD.EXE
              If executed from outside a batch script, if will quit CMD.exe

   exitCode   Sets the %ERRORLEVEL% to a numeric number.
              If quitting CMD.EXE, set the process exit code no.

To close an interactive command prompt, the keyboard shortcut ALT + F4 is an alternative to typing EXIT.

Errorlevel

EXIT /b has the option to set a specific errorlevel, EXIT /b 0 for sucess, EXIT /b 1 (or greater) for an error.
The exit code can be an integer of up to 10 digits in length (positive or negative).

EXIT without an ExitCode acts the same as goto:eof and will not alter the ERRORLEVEL

n.b. You should never attempt to directly write to the %ERRORLEVEL% variable, (SET ERRORLEVEL n ) instead use EXIT /b n as a safe way to set the internal ERRORLEVEL pseudo variable to n.

When EXIT /b used with FOR /L, the execution of the commands in the loop is stopped, but the loop itself continues until the end count is reached. This will cause slow performance if the loop is (pointlessly) counting up to a large number.
In the case of an infinite loop, this EXIT /b behaviour will cause the script to hang until manually terminated with Ctrl + C

Exiting nested FOR loops, EXIT /b can be used to exit a FOR loop that is nested within another FOR loop.
This will only work if the inner FOR loop is contained in a separate subroutine, so that EXIT /b (or goto:eof) will terminate the subroutine.

Examples

Exit if a required file is missing:

@Echo Off
If not exist MyimportantFile.txt Exit /b
Echo If we get this far the file was found

Set the errorlevel to 5:

@Echo Off
Call :setError
Echo %errorlevel%
Goto :eof

:setError
Exit /B 5

Use EXIT /b to exit a nested FOR loop (so skipping the values X,Y and Z), but still continue back to the main outer loop:

@Echo Off
Setlocal 
For %%A in (alpha beta gamma) DO (
   Echo Outer loop %%A
   Call :inner 
)
Goto :eof

:inner
For %%B in (U V W X Y Z) DO (
   if %%B==X ( exit /b 2 )
   Echo    Inner loop    Outer=%%A Inner=%%B
)

EXIT is an internal command.
If Command Extensions are disabled, the EXIT command will still work but may output a spurious 'cannot find the batch label' error.

“Making music is not about a place you go. It’s about a place you get out of. I’m underwater most of the time, and music is like a tube to the surface that I can breathe through. It’s my air hole up to the world. If I didn’t have the music I’d be under water, dead” ~ Fiona Apple

Related:

VERIFY - Provides an alternative method of raising an errorlevel without exiting.
TSKILL - End a running process.
Powershell: Exit - Exit Powershell or break - Exit a program loop.
Equivalent bash command (Linux): break - Exit from a loop.