Compilation error

Arduino: 1.6.12 (Windows 10), Board: "Arduino/Genuino Uno"

In function 'void loop()':

demo:9: error: 'ledPin' was not declared in this scope

digitalWrite(ledPin, HIGH);

^

exit status 1 'ledPin' was not declared in this scope

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'ledPin' was not declared in this scope

Where is it declared ?

If only you had posted your program ...