In alternating current (AC) circuits, impedance is a term that includes both resistance and reactance. In direct current (DC) circuits, impact resistance is the term used to describe the manner in which electrical energy is converted into heat. In DC circuits, the resistivity of materials will differ. However, when it comes to AC circuits, the current oscillates as a sine wave which is constantly changing. As a result, inductance and capacitance come into play. Inductance causes a small back current to be produced which reduces the overall current. Capacitance is the charge stored across a capacitor divided by the potential difference across it.
Impedance, then, is a more general term for resistance that includes reactance. Impact resistance does not change. Impedance is the result of both resistance and reactance and its value varies with frequency.