The natural phenomenon characterized by the rapid expansion of gases is called an explosion. Explosions occur when a sudden release of energy generates a large amount of gas and heat, leading to a rapid increase in pressure. This can result from chemical reactions, such as those in explosives, or physical changes, such as the sudden release of compressed gas. Explosions can have significant effects, including destruction and fires, making understanding their principles essential in fields like safety engineering and hazardous materials management.