AC Impedances are complex numbers of the form
Z = R + jX(),
where =2f.

Resistance (R): The real (frequency invariant) portion of Z. The basic unit of resistance is the ohm ().

Reactance (X): The imaginary (frequency variant) portion of Z. Reactances may also be expressed in terms of imaginary ohms (j). More commonly they are expressed as either an inductance (L) or a capacitance (C).

Inductance (L): If X is positive in sign, it is inductive. The inductance can be calculated from Z = j L where = 2f. The basic unit of inductance is the Henry (H); commonly high frequency inductances are specified in terms of nanoHenrys (nH).

Capacitance (C): If X is negative in sign, it is capacitive. The capacitance can be calculated from Z = 1/(jC) where = 2f. The basic unit of capacitance is the Farad (F); commonly high frequency capacitances are specified in terms of picoFarads (pF), often pronounced "puff" by microwave engineers.

Admittance (Y): The inverse of an impedance (i.e. the ratio of current to voltage) is called the admittance. Complex admittances take the form
Y = G + jB(t)

Conductance (G): The real (frequency invariant) portion of Y. The basic unit of conductance is the Siemen (S), and was formerly the mho.

Susceptance (B): The imaginary (frequency variant) portion of Y is called the susceptance. Reactances may also be expressed in terms of imaginary Siemens (jS).

Immittance: A general term for both impedance and admittance, used when the distinction is irrelevant.

The equation relating voltage, current, and impedance (resistance) is known as Ohm's law:
V = I R

See also:
Gain, Generator Impedance, Impedance, Impedance Matching, Load Impedance, Reflection Coefficient, S-Parameters