What is defined as the malicious and intentional act of starting a fire?

The definition of the malicious and intentional act of starting a fire aligns perfectly with the term "arson." Arson involves not just the act of setting a fire, but specifically doing so with the intent to cause damage or harm. This definition encompasses the need for both intent and action, distinguishing arson from other fire-related terms.

Combustion refers to the chemical reaction that occurs when a substance reacts with oxygen to produce heat and light, which does not imply malice or intention. An explosion generally describes a rapid expansion of gas resulting in a violent release of energy, and while it might involve fire, it is not the act of starting a fire intentionally. A firestorm refers to a large, intense fire that can create its own wind system and does not relate to the deliberate initiation of fire.

Thus, the precise nature of arson as an intentional and harmful act categorizes it distinctly, making it the correct answer to the question regarding the definition of starting a fire with malice.