Ground fault circuit interrupter, also referred to GFCI, GFI, or a residual-current device. This devices detects a leak in current, or an imbalance between the current on the hot and neutral wires. This is used to prevent electrical shock.
GFCI breakers are often required where electricity can contact water or some other ground, including kitchens, bathrooms, and outdoors. The breakers may be installed within the breaker panel or as an outlet.
See also afci which is similar in appearance but used to protect against arcing that could cause a fire.