Electrical impedance
Impedance is a general term describing something that slows a movement down. In electronics it refers to any effect that prevents an electric current from reaching an infinite level.
Impedance (symbol Z) is measured in ohms. In a direct current circuit, impedance is due to resistance, whereas in an alternating current circuit impedance may be also due to inductive or capacitive reactance (from an inductor or capacitor respectively).
Impedance in an RLC series circuit is the ratio of RMS voltage to RMS current and is given by the formula
Z = VRMS / IRMS = sqrt( R2 + (XL - XC)2 )
where R is the resistance, XL is the inductive reactance, and XC is the capacitive reactance.
When fitting components together to carry electromagnetic signals, it is important to match impedance to prevent signal loss. Failing to do so results in impedance mismatch, which can be avoided with various matching devices.
For example, a conventional radio frequency antenna for carrying broadcast television in North America was standardized to 300 ohms, using balanced, unshielded, flat wiring. However cable tv systems introduced the use of 75 ohm unbalanced, shielded, circular wiring, which could not be plugged into most TV sets of the era. To use the newer wiring on an older TV, small devices known as baluns were widely available. Today most TVs simply standardize on 75 ohm feeds instead.