A dead short is a short circuit that creates no real resistance, allowing the amperage to build up until it trips the circuit breaker. These can be difficult to track down and diagnose because the current builds so rapidly that it usually trips the breaker immediately.
Dead shorts can be dangerous due to the high amount of current the situation creates. Repeatedly cycling the breaker to try to locate the short can cause damage to the wiring and present a major fire hazard.
Common causes of a dead short include a direct connection between the positive and negative power legs or a direct connection between the positive and ground wires without any load or resistance in between.