An inverter is an electronic component used in digital systems that takes the logical value of its input and complements it. So, if the input voltage is logically low, a high voltage appears on the output, and vice versa. The exact values of the high and low voltages depend on the semiconductor family. See CMOS, TTL.

Here are a few symbols for the same piece of hardware, depending upon the desired use. Also, a polarity indicator (wedge) might replace the bubble.

Hi -> Lo         Lo -> Hi           In | Out
                                   ----+----
 |\                |\                H |  L
-| \o-           -o| \--             L |  H
 | /               | /
 |/                |/

Power Inverter

A nifty little box that most commonly turns a car's DC output into AC so that you can run things like TVs, laptop computers, or camcorders from the car. They commonly plug into a car's cigarette lighter socket, but there are a few that come with battery clamps.

To build a power source, attach an inverter to ground. Then, attach the load between the inverter and ground.
The inverter will turn the zero volt ground source into +5 volts, and you can power off the difference between ground and the inverter output. If it doesn't work, you can burn down your local (insert name of electronics store) and plead insanity, thus tieing up all the remaining loose ends (so to speak).

An inverter is also an electronic device that converts DC to AC, for example 12 V DC to 120 V AC. It is also called a DC/AC converter, and is used in UPS:es, and for powering household appliances from a battery and/or solar panel.

The simple inverters produce an output waveform that is almost a square wave. This is simple and efficient, but may not work for more demanding devices, such as computers, computer monitors or hifi systems. The more advanced inverters produce a pure sine wave. Typically, the efficiency is 85 - 95 %.

It is important to remember that an inverter increases the voltage, but not the power, so the output current is proportionally lower.

Example: You have a 12 V battery and want to power a 120 V, 60 W lightbulb. The current through the lightbulb will be 60 / 120 = 0.5 A. The battery needs to supply the same power, 60 W. Therefore the current from the battery will be 60 / 12 = 5 A, and even slightly more because of losses in the inverter.

I have not been able to find a good origin of the word inverter. A possible explanation is that it inverts the actions of a power supply.