preparing an analog signal before digitizing it: amplifying the signal, removing frequencies above half the sampling rate (the Nyquist frequency), linearizing it, and so on, although digital processing now can perform linearization and other traditional signal conditioning tasks more effectively than analog circuitry
Imagine feeding the output of a temperature sensor, which is in millivolts, to an Analog-to-digital converter to be processed. Is it possible for the Analog-to-Digital converter to process such a minute voltage amplitude? The answer is probably no.
To process the form or mode of a signal so as to make it intelligible to, or compatible with, a given device, including such manipulation as pulse shaping, pulse clipping, compensating, digitizing, and linearizing.