Resistance R in a DC circuit is expressed by Ohm's law as a ratio of voltage to current. Extending this to alternating current, the ratio of voltage and current is called impedance (Z). So far, so familiar.

If we divide the impedance into a real part and an imaginary part, the real part is the resistance. So what is the imaginary part? This is called Reactance X.

Expressed as a formula, Z = R + jX.

j is √(-1) in imaginary units. An ideal inductor has a reactance of ωL.

The unit of impedance is Ω (ohm). Ω is the only Greek letter in the SI unit symbol. This unit is named after the German physicist Georg Simon Ohm, who discovered Ohm's law. It seems that

The reciprocal of impedance is called admittance Y. Its real part is called the conductance G and its imaginary part is called the susceptance B.

Expressed as a formula, Y = G + jB.

An ideal capacitor has a susceptance of ωC.

The unit of admittance is S (Siemens). In the past, it was sometimes called mho, and the symbol was an upside-down Ω.

As an aside, the lowercase s in S stands for seconds in time. Be careful not to accidentally write nanoseconds as nS. It will become Nano Siemens.