What do you call a "de-amplifier" device(is there such a thing)?
Such a device could be called an attenuator (defined as "an electronic device that reduces the power of a signal without appreciably distorting its waveform.")[1].

For specialized RF situations, there are specific designs based usually around passives of carefully-selected values for a given frequency or frequency range.

For low-frequency applications, or where extreme precision isn't necessary, it's often common (at least for hobbyists, not sure about professional applications) to use simple voltage dividers with the divided voltage connected an an analog input, for example on a microcontroller.