The concept of impedance generalizes Ohm’s law to AC circuit analysis. Unlike resistance, the impedance of an electric circuit is a function of frequency:
Impedance (Z) at different frequencies:
The example below shows impedance of an RC network changes over frequencies.
In the op-amp datasheet, there are two kinds of op-amp input impedance. One is common mode input impedance, the other one is differential input impedance. The picture below shows the common mode input impedance model. For differential input impedance, it is the ratio of the differential input voltage change to the input bias current change.
Below is an example of the MCP601 common and differential modes input impedance specification.