Resistance (or impedance).

Obviously it's not infinite impedance--that would mean it soaks up all the voltage. It can't be zero impedance either...that would be a superconductor, and there aren't many of those around. Voltage sources have impedance, but it's not infinite. Sometimes the impedance of the source is critical--radio frequency amplifiers have to be impedance matched to their transmitting antennas if you don't want to burn them out.

impedance

Resistive

Or a resistor

High Impedance

A differential input stage employing matched pairs of field effect transistors provides a high input impedance, good dc stability and is sufficiently simple and robust for student use.