analogread returns values

I just started messing around with Arduino Leonardo and have been working with a photoresistor. while i do get a value returned to the serial monitor, I don't believe the value to be correct. I disconnected Leonardo from the circuit, and still get a serial monitor return of around 300. attached is my code. am i doing something wrong or am i just missing something? Thanks !

lnt lightSensor = A0;
int light_val;

void setup() {
Serial.begin(9600);
while (!Serial);//wait untill serial is ready
Serial.println ("standby");
}

light_val = analogRead(lightSensor); // read the value from the photosensor
Serial.print("light sensor reads ");
Serial.println( light_val );
delay(1000);

}

I disconnected Leonardo from the circuit, and still get a serial monitor return of around 300

It's called a floating pin.
Absolutely nothing wrong.

Please use code tags when posting code, and post all your code.