A capacitor, sometimes called a condenser, is a passive electronic component that stores electric charge. There are many different forms of capacitors, but each one has at least two conductors separated by a non-conductor. Conductors are similar to batteries, but are smaller, lighter, and charge much quicker.

Capacitors are made from two metal plates, called conductors, that are on top of and near one another, but do not touch. The conductors require a lot of surface area to provide capacitance (the ability to store an electric charge), so capacitors are often rolled or folded into another shape (like a cylinder). These conductors are separated by an insulator called a dielectric. The size and type of dielectric determines whether the capacitor is better suited for high frequency or high voltage applications. For example, capacitors with Mylar dielectric are most commonly used in timer circuits like those in clocks or alarms, while capacitors with ceramic dielectric are most commonly used in high-frequency purposes like X-ray and MRI machines. A capacitor allows alternating current (AC) to pass, but blocks direct current (DC) signals. Because of this property, capacitors are often used to separate the AC and DC parts of a signal. This use is called capacitive or AC coupling.

There are many ways that capacitors can be used, and at least one is included in almost every electrical product. One way a capacitor can be used is for energy storage as a sort of temporary battery because capacitors can store electric energy even when disconnected from their charging circuits. They are often used to maintain power supply while the batteries of electronic devices are charged. Capacitors can also be used to provide large pulses of current for pulsed power applications such as pulsed lasers, radar, and particle accelerators. Reservoir capacitors (large capacitor banks) are also used for detonators in nuclear weapons and some other types of specialty weapons. There are many possible uses of a capacitor.

Although useful, capacitors can be very dangerous because they sometimes retain their charge long after the power source has been removed from the circuit. If the wrong part of an electronic device is touched, the stored charge in the capacitor can cause a painful or even fatal electric shock. Electronics often contain instructions and warnings to avoid this danger. When working with equipment containing larger capacitors often requires special wrist straps to be worn to protect both the user and the equipment.

While there are risks when using capacitors, they are also very useful and are integral to modern technology.