A short circuit (sometimes abbreviated to "short" or "s/c") is an electrical circuit that allows an electric current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit.

The opposite of a short circuit is an open circuit, which is an infinite resistance (or very high impedance) between two nodes.

A short circuit is an abnormal connection between two nodes of an electric circuit intended to be at different voltages. This results in a current limited only by the Thévenin equivalent resistance of the rest of the network which can cause circuit damage, overheating, fire or explosion. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, for the purpose of voltage-sensing crowbar circuit protectors.

In circuit analysis, a short circuit is defined as a connection between two nodes that forces them to be at the same voltage. In an 'ideal' short circuit, this means there is no resistance and thus no voltage drop across the connection. In real circuits, the result is a connection with almost no resistance. In such a case, the current is limited only by the resistance of the rest of the circuit.

A common type of short circuit occurs when the positive and negative terminals of a battery or a capacitor are connected with a low-resistance conductor, like a wire. With a low resistance in the connection, a high current will flow, causing the delivery of a large amount of energy in a short period of time.

A high current flowing through a battery can cause a rapid increase of temperature, potentially resulting in an explosion with the release of hydrogen gas and electrolyte (an acid or a base), which can burn tissue and cause blindness or even death. Overloaded wires will also overheat causing damage to the wire's insulation, or starting a fire.

In electrical devices, unintentional short circuits are usually caused when a wire's insulation breaks down, or when another conducting material is introduced, allowing charge to flow along a different path than the one intended.

In mains circuits, short circuits may occur between two phases, between a phase and neutral or between a phase and earth (ground). Such short circuits are likely to result in a very high current and therefore quickly trigger an overcurrent protection device. However, it is possible for short circuits to arise between neutral and earth conductors and between two conductors of the same phase. Such short circuits can be dangerous, particularly as they may not immediately result in a large current and are therefore less likely to be detected. Possible effects include unexpected energisation of a circuit presumed to be isolated. To help reduce the negative effects of short circuits, power distribution transformers are deliberately designed to have a certain amount of leakage reactance. The leakage reactance (usually about 5 to 10% of the full load impedance) helps limit both the magnitude and rate of rise of the fault current.