Impedance

Impedance describes a measure of opposition to a sinusoidal alternating current. Impedance (Z) is found by summing the squares of the resistance (R) and total reactance (X).

Z=sqrt{X^2+R^2}

X=sqrt{R^2-Z^2}

R=sqrt{X^2-Z^2}

More info: http://en.wikipedia.org/wiki/Electrical_impedance