What is meant by ADC saturation?

An ADC has a measurable input voltage range that it can produce valid samples for. For example, an ADC may be able to produce 16 bit samples on the range [-.5V,+.5V]. A 16 bit range will give codes from [-32,768,32,767], which are equal to the [-.5V,+.5V] input range. If an input goes outside of the valid voltage range (but still within allowable electrical ratings), the ADC will 'saturate', in which it will continue to feed out the maximum or minimum 16 bit values. This will continue until the input comes back into the measurable range.