**Resistance** is the measure of opposition to the current flow offered by the material.Usually denoted by R. Resistance is when voltage & current stay in phase, and it follows basic Ohms law. It is considered as 'real'.

**Reactance ** is the resistance offered to the ac currents by inductors and capacitors only. Usually denoted by X.

For capacitors X=1/(2πfC); where f is the frequency,C is the capacitance.

For inductors X= 2πfL; where f is the frequency,:L is the inductance.

Reactance is when Voltage & current are 90deg out of phase. Depending on whether it is +90 (lead) or -90 (lag) it is called an inductive reactance or capacitive reactance. Reactance is normally denoted by the imaginary operator 'j' .

**Impedance** is the sum of the resistance and reactance of a circuit denoted by Z=R+jX (for primarily inductive cirucits) or Z=R-jX (for primarily capacitive circuits).

where j=√(-1).

It is usually represented as a complex number like 2 - j3.5. The phase angle between V & I could be any value.