It involves Quantisation which is the process of mapping continuous infinite values to a smaller set of discrete finite values.

It is the rounding error between the actual analogue input and the digitised value.

Quantisation error is introduced due to Nyquist Sampling technique.

It is the difference between the analogue signal and the closest available digital value at each sampling instant from the ADC (Analogue to Digital Converter).

It is referred to as the noise introduced by quantisation in an ideal ADC (Analog to Digital Converter).