Recall that the electrical impedance is defined as the voltage drop across an element divided by the current through the element. For a (simple geometry) piezoelectric element, the electrical impedance over a given frequency range will appear similar to that shown here:

The presence of electrical resonances and anti-resonances make the piezoelectric impedance unique. The resonances result from the electrical input signal exciting a mechanical resonance in the piezo element. For each mechanical resonance in the piezo element, a resonance/anti-resonance pair will exist in the impedance.

This characteristic piezoelectric impedance can be modeled by an equivalent circuit.