1. A circuit that compares two input signals, which may be digital or analog, but that always results in a digital-signal output.
2. A comparator in which movement results in a change in some electrical quantity that is then amplified by electrical means.

A comparator is a device used to inspect a gaged part (thickness of a metal sheet, a rod, or a wire) for deviation from a specified dimension, by mechanical, electrical, pneumatic, or optical procedures.