What is a Gigafactory?

Tesla built their first Gigafactory in Nevada, United States. CEO Elon Musk announced that they're planning multiple new Gigafactories, maybe even two in Europe. In this Gigafactory Tesla will produce lithium-ion battery cells for their cars. According to the Tesla website the name Gigafactory comes from the factory’s planned annual battery production capacity of 35 gigawatt-hours (GWh).