why an operational amplifier is called differential amplifier not a difference amplifier

 

A differential amplifier is a configuration of op amp that amplifies a difference between two input voltages. It is a little vague but my understanding is that a differential amplifier is the general configuration, reference All About Circuits - Volume 3:Chapter 8, whereas a difference amplifier is a special case where each resistor in the feedback network of the differential amplifier is equal, thus giving an output that is a pure representation of the difference between the inputs, i.e. Vout = V2 - V1

It is common practice to use the name differential and difference interchangeably when referring to op amps.

Dave