For op-amps, while the voltage at the inputs should be the same, in reality, they never are. They’re going to be off by at least a little, and depending on the op-amp, it could be better or worse. This difference is called the offset voltage.
The differential DC voltage required between the inputs to force the output to zero volts.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
The input offset voltage
is a parameter defining the differential DC voltage required between the inputs of an amplifier, especially an operational amplifier (op-amp), to make the output zero (for voltage amplifiers, 0 volts with respect to ground or between differential outputs, depending on the output type).
An ideal op-amp amplifies the differential input; if this input difference is 0 volts (i.e. both inputs are at the same voltage), the output should be zero. However, due to manufacturing process, the differential input transistors of real op-amps may not be exactly matched. This causes the output to be zero at a non-zero value of differential input, called the input offset voltage.