An op-amp configured to differentiate the signal that is coming in. Much as a differential measures the angle of a curve in calculus, a differentiator will output how much the input is changing. For example, if the input is steady, the output will be zero. If the input is changing linearly, the output will be a constant equal to the rate of change.
An op-amp differentiator simulates mathematical differentiation, which is a process of determining the instantaneous rate of change of a function.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
In electronics, a differentiator is a circuit that is designed such that the output of the circuit is approximately directly proportional to the rate of change (the time derivative) of the input. A true differentiator cannot be physically realized, because it has infinite gain at infinite frequency. A similar effect can be achieved, however, by limiting the gain above some frequency.