Why Impedance Z is the complex opposition to AC, combining resistance and reactance. Its reciprocal, Y = 1/Z, is admittance, measured in siemens, and it tells you how readily current flows for a given applied voltage. Admittance itself splits into a real part (conductance) and an imaginary part (susceptance), just as impedance splits into resistance and reactance.
Watch out Conductance is only the reciprocal of resistance, the real part of the story, and susceptance is the reactive part of admittance; reluctance is a magnetic-circuit term, the opposition to magnetic flux, not an AC electrical quantity.
Z and Y are the whole-package pair: impedance flips to admittance. R flips to G, X flips to B.