Final answer:
Flashover is the term used to describe simultaneous ignition caused by a gradual heating of all contents of a fire area. It occurs when the surrounding materials reach their ignition temperature simultaneously, resulting in a sudden and intense fire spread.
Step-by-step explanation:
Simultaneous ignition caused by a gradual heating of all contents of a fire area is called flashover. Flashover occurs when the heat from the fire causes the surrounding materials to reach their ignition temperature simultaneously, resulting in a sudden and intense fire spread. This phenomenon is often observed in enclosed spaces, such as rooms or buildings, where the heat and gases generated by the fire cannot escape easily.