Flashover is a phenomenon that occurs during a fire in an enclosed space or compartment, where every combustible surface exposed to thermal radiation rapidly and simultaneously ignites. Flashover typically occurs when the upper portion of the compartment reaches a temperature of approximately 1,100 °F for ordinary combustibles. The heat transfer at and beyond flashover is primarily by radiation. Flashover is one of the most-feared phenomena among firefighters, and they are taught to recognize the signs of imminent rollovers and flashovers and to avoid backdrafts.

Factors that influence a flashover event include the location of the fire, size, volume, and shape of the compartment, fire growth rate, contents and their exposed surfaces, and compartment ventilation characteristics. Responders exposure to flashover can be influenced by arriving on the scene at a pre-flashover state, compartments built tighter with fewer sources of air leakage, bunker gear providing a false sense of security, and rooms filled with many more synthetics that flash at lower temperatures.

There are different types of flashover, including hot rich flashover, backdraft, and delayed flashover. Hot rich flashover occurs when the upper area of the room is saturated with heat, which radiates downward to all combustibles in the space, causing the rooms contents to simultaneously reach the ignition point. Backdraft is the least common type of flashover that firefighters encounter, and it occurs when a fire starts in a back portion of a structure, such as a rear bedroom in a house. The smoke and energy travel throughout the home, filling it with superheated gases. When firefighters enter the building, a hot rich flashover can occur. Delayed flashover occurs when a fire starts in a back portion of a structure, such as a rear bedroom in a house. The smoke and energy travel throughout the home, filling it with superheated gases. When firefighters enter the building, a hot rich flashover can occur.