An Analog-to-Digital Converter (ADC) is monotonic if, for increasing analog voltage input, the digital output code increases and vice versa. Monotonic behavior does not guarantee that there will be no missing codes.
Monotonic behavior is an especially important characteristic for ADCs used in feedback control loops since non-monotonic response can cause oscillations in the system. Monotonicity is a critical specification with automatic control applications. It does not, however, mean no missing codes.
A monotonic ADC can still have a DNL greater than 1 LSB which would result in a missing code(s) at some point in the transfer function.
Non-Monotonic ADC with Missing Code
A non-monotonic ADC transfer function will miss a lower code until after the higher code is converted (assuming linearly increasing input voltage). The figure below shows a 3-bit ADC with a missing code: