Literally electricity that is not in action, otherwise known as static electricity, which is technically an electrostatic charge. Anytime one surface or point has an electric charge relative to another you have electrostatic energy, which is a form of potential energy. Electrostatic is also a type of transducer design. Most widely employed in loudspeakers, electrostatics (as they are often called) are built somewhat like a large capacitor. There are two plates, one of which can move. A DC bias voltage is applied to them to create an electrostatic charge. Then the AC audio signal is applied, and the interaction between the resulting (alternating) magnetic field and the electrostatic field forces the movable plate to move back and forth. Some audiophiles consider this a more pure and better sounding method of reproducing audio than our typical moving coil dynamic loudspeaker designs. However most tend to be quite expensive and it is debated whether there is an overall improvement in sound – many electrostatic speakers are characterized as having little or no bass punch. In fact separate subs of standard moving coil design are often employed as a workaround for this.