Impact resistance relates to direct current (DC), whereas impact reactance is a concept that applies to alternating current (AC). Impact resistance is the amount of resistance offered by a conductor in a direct current circuit. The resistance is due to the electrons within the conductor impacting with the forces of attraction between oppositely charged ions (ionic lattice). This results in conversion of electrical energy into heat.
Impact reactance applies to AC currents, which operate as sine waves. Since sine waves are constantly changing, they bring other factors into the resistance equation. The conductor of an AC current produces a small back current, which acts as a resistance. Another factor, called capacitance also affects AC currents. Capacitance relates to the amount of charge stored across a capacitor divided by the potential difference across it. Reactance is a measure of the amount of opposition to AC current based on these two factors.