What is monolithic ADC?

a monolithic ADC is an integrator-based analog-to-digital converter. Simply put, the input voltage signal is integrated, compared to a reference voltage, and converted into a digital representation. The integrator uses a reference capacitor (hence monolithic i imagine), which is chosen depending on the bandwidth of the input signal.

Don't take my Word for it, see for yourself. The ICL7135 for example is a monolithic ADC with 4 and 1/2 digits BCD output.