阻抗 （ electrical impedance ）是 电路
中 电阻
、 电感
、 电容
对 交流电
的阻碍作用的统称。（定义来自wikipedia）
阻抗为什么使用复数呢？请看下面的描述：
It is necessary to introduce the concept of impedance in AC circuits because
there are other mechanisms impeding the flow of current besides the normal
resistance of DC circuits.
我们把阻抗的定义引人到交流电中是必要的，因为当电流流过电线时，不仅存在在直流电中的电阻，同时还存在另外两种电抗。
There are an additional two impeding mechanisms to be taken into account in AC
circuits:
这两种电抗是：
the induction of voltages in conductors self-induced by the magnetic fields of
currents ( inductance ),
在导体中，通过电流磁场形成的电感
and the electrostatic storage of charge induced by voltages between conductors
( capacitance ).
在导体之间因为电压而存储的电荷
The impedance caused by these two effects is collectively referred to as
reactance and forms the imaginary
part of
complex impedance
这两种效应合在一起被称为（电抗），同时也是复数的虚部
whereas resistance forms the real part.
而电阻则是虚数的实部。
Z is the impedance , measured in ohms.
R is the resistance 电阻, measured in ohms.
X is the reactance 电抗, measured in ohms.
Y is the admittance , measured in siemens (a.k.a. mho, the inverse of
ohm).
G is the conductance 导纳, measured in siemens.
B is the susceptance 电纳, measured in siemens.
http://zh.wikipedia.org/wiki/%E9%98%BB%E6%8A%97
http://en.wikipedia.org/wiki/Electrical_impedance