Mixed means put together, such as RF-Analog and Analog-Digital.
For example, LNA is a mixed-signal RF-Analog component. RF being that is placed at RF front-end and Analog being that is basically an analog component - a differential amplifier. Mixer, PA and duplexer too are RF-Analog components.
ADC is mixed-signal Analog-Digital because it has a digital component inside it which is a DAC.
Mixed Signal Intergarted Circuit means the circuit have the device that process the Analog and Digital data which are put on the same chip together. For example, DAC or ADC circuit that process analog to digital or digital to analog. Or even like in the optical communication, the front part like Transimpedance amplifer and Limiting amplifer works in the analog way and the end part like CDR works in the digital way.