The transitional phase in a fire when the radiant heat ignites the flammable gases within an enclosed structure. This will look like the fire has spread quickly across the thick smoke on the ceiling, and at this point whatever the flame touches catches fire in a matter of seconds. The only warning sign other than the incredible heat at the source of the fire, and the plumes of smoke start to ignite and this is called rollover. These rollovers can exist for a few seconds before flashover.
Firefighters try to prevent a flashover from occurring by cooling the surrounding air with bursts of water mist sprayed into the air above the fire. The burst allows for the most amount of water to be converted into vapor, cooling a wider area than the spray off the nossle alone.
When this phenomenon occurs outside, it's referred to as a firestorm.