A special type of AC circuit breaker designed for increased safety. A GFI compares the current on the hot AC lead to the neutral AC lead. If there is any difference then that means there is a problem and the current should be shunted to ground. A GFI breaker is designed to VERY quickly open and interrupt the power should this condition arise. This is better than simply shunting dangerous current to the safety ground because even when current is going to ground in a normal system the breaker will not open until its current capacity is surpassed. If you happen to be in the path of this current then…well…that would be bad. A GFI interrupts the current flow before significant amounts try to find a path to ground.