A refrigerator (colloquially fridge) is a common household appliance that consists of a thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that transfers heat from the inside of the fridge to its external environment so that the inside of the fridge is cooled to a temperature below the ambient temperature of the room. The lower temperature lowers the reproduction rate of bacteria, so the refrigerator reduces the rate of spoilage. Optimum temperature range for perishable food storage is 3 to 5 °C (37 to 41 °F). A refrigerator is sometimes referred to as an icebox in American usage.