In an operational amplifier, the output voltage should theoretically be 0 V when the voltage differential between the input pins is 0 V. However, in reality, VBE of the positive and negative input transistors in the input stage's differential amplifier circuits differs. As a result, the voltage differential between the input pins is not 0; in fact, a slight amount of voltage is output. This output voltage is known as offset voltage.
Also refer to FAQ opcomp-0002 "What is input offset voltage (VIO) ?"
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