There has been a lot of confusion in the past with terms such as flashover, backdraft, roll over etc.
In this section I will try to describe the term flashover from a firefighters perspective.

When a fire is burning in a compartment, most of the smoke is unable to escape and begins to accumulate. Because the smoke is heated, it is buoyant and will rise to the ceiling. As the amount of smoke increases the smoke level will get lower and denser. It is vital to understand that the smoke contains a certain amount of "un-burnt fuel", so as the smoke accumulates, so does the amount of un-burnt fuel vapour. Eventually there will be enough un-burnt fuel to form a flammable mixture.