The defining property of a perfect conductor is that static electric field and the charge density both vanish in its interior.
An external electric field is screened from the interior of the material by rearrangement of the surface charge.
[1] Alternatively, a perfect conductor is an idealized material exhibiting infinite electrical conductivity or, equivalently, zero resistivity (cf.
Another example is electrical circuit diagrams, which carry the implicit assumption that the wires connecting the components have no resistance.
Yet another example is in computational electromagnetics, where perfect conductor can be simulated faster, since the parts of equations that take finite conductivity into account can be neglected.