ITER (initially the International Thermonuclear Experimental Reactor, "iter" meaning "the way" or "the path" in Latin) is an international nuclear fusion research and engineering megaproject aimed at replicating the fusion processes of the Sun to create energy on earth. Upon completion of construction and first-plasma planned for 2026, it will be the world's largest magnetic confinement plasma physics experiment and the largest experimental tokamak nuclear fusion reactor, which is being built next to the Cadarache facility in southern France. ITER will be the largest of more than 100 fusion reactors built since the 1950s, with ten times the plasma volume of any other tokamak operating today.The purpose of ITER is to demonstrate the scientific and technological feasibility of fusion energy for future electricity generation. ITERs goals are: to produce 10 times as much output energy as input for short time periods; to demonstrate and test technologies that would be needed to operate a fusion power plant including cryogenics, heating, control, and diagnostics systems, including remote maintenance; to achieve and learn from a burning plasma; to test tritium breeding; and to demonstrate the safety of a fusion plant.ITER's thermonuclear fusion reactor will attempt to use 50 MW of heating power to create a plasma of 500 MW (thermal) for periods of 400 to 600 seconds. This would mean a ten-fold gain of plasma heating power or, as measured by heating input to thermal output, Q ≥ 10. The current record for energy production using nuclear fusion is held by the Joint European Torus reactor, which injected 24 MW of heating power to create a 16 MW plasma, for a Q of 0.67. Beyond just heating the plasma, the total electricity consumed by the reactor and facilities will range from 110 MW up to 620 MW peak for 30-second periods during plasma operation. As a research reactor, the heat energy generated will not be converted to electricity, but simply vented.ITER is funded and run by seven member parties: the European Union, China, India, Japan, Russia, South Korea, and the United States; the United Kingdom and Switzerland participate through Euratom, while the project has cooperation agreements with Australia, Kazakhstan, and Canada.Construction of the ITER complex started in 2013, and assembly of the tokamak began in 2020. The initial budget was close to €6 billion, but the total price of construction and operations is projected to be €18 to €22 billion; other estimates place the total cost between $45 billion and $65 billion, though these figures are disputed by ITER. Regardless of the final cost, ITER has already been described as the most expensive science experiment of all time, the most complicated engineering project in human history, and one of the most ambitious human collaborations since the development of the International Space Station (€100 billion budget) and the Large Hadron Collider (€7.5 billion budget).ITER's planned successor, the EUROfusion-led DEMO, is expected to be one of the first fusion reactors to produce electricity in an experimental environment.