A reactor is a large, specially designed container or facility where controlled chemical or nuclear reactions take place. The most well-known type is a nuclear reactor, which splits atoms to release enormous amounts of energy that can generate electricity for millions of homes.

Inside a nuclear reactor, scientists and engineers carefully manage a chain reaction involving uranium or other radioactive materials. The reaction produces intense heat, which boils water into steam that spins giant turbines to create electrical power. Nuclear reactors power submarines and aircraft carriers, letting them travel for years without refueling. They also provide about 10% of the world's electricity.

The word reactor can also describe containers used in chemical plants and laboratories where reactions happen under controlled conditions. A bioreactor grows living cells or microorganisms for making medicines or treating waste.

What makes something a reactor rather than just any container is the control: engineers design reactors to keep reactions at exactly the right speed and temperature, with safety systems to prevent accidents. A campfire might involve a chemical reaction, but it's not a reactor because you're not precisely controlling what's happening at a molecular level. In a reactor, every aspect of the reaction is measured, monitored, and managed.