What is meant by the term “op-amp input offset voltage”?

In an ideal operational amplifier, if both the inverting and non-inverting inputs are held at the exact same voltage (zero differential), the output will be exactly zero volts. However, real-world op-amps are built with internal transistors that are never perfectly matched due to tiny manufacturing variations. Because of these slight internal imbalances, a real op-amp will produce a small, non-zero output voltage even when both inputs are shorted together or grounded. To correct this, a tiny differential DC voltage must be applied between the two input terminals to balance the internal circuitry and force the output exactly to zero. This corrective differential voltage is known as the 'input offset voltage.'