Arduino has an input value in disconnected pin
I wish to measure the voltage of a 1.5V battery using a microcontroller (supply voltage of 3V).
I thought of connecting the battery on the ADC4 input of the microcontroller, and wrote a program which reads the ADC4.
Initially, I programmed the microcontroller without connecting the battery to ADC4. I was expecting to get a value of 0V (nothing connected to ADC4), but I didn't. Instead, I am getting a higher value which is also fluctuating. I tried to measure the voltage on ADC4 with a voltmeter, but I can't get a reading since it is obviously fluctuating. Why is that happening?