A particular type of operational amplifier (op-amp) which has an output that is either on or off. It is a type of switch, but has two inputs, one for what is being measured and the other being a reference point. The comparator will normally switch on when the input is less than the reference. An example would be in a thermostat. Then input would be given by a thermistor and the reference would come from a variable resistor. This allows the actual switching temperature to be altered.