It is defined as a constant difference, over the whole range of the ADC, between the actual output value and the ideal output value.
Gain error:
It is defined as the difference of the slope of the actual output values and the ideal output values.
Non-linearity error
For an ideal ADC, the output is divided into 2n uniform steps each with the width ?. Any deviation from the ideal step width is the Differential Non-Linearity. (DNL)