Ignition is the rapid oxidation of substances in the exothermic chemical process of combustion, which releases heat, light and various reaction products. This definition does not include slower oxidation processes such as rust or digestion.

The fire is very hot, because the weak double bond in molecular oxygen O2 is converted into the strong bond in the combustion product, carbon dioxide and water release energy (418 kJ / 32 g O2); the bond energy of fuel only plays a small role here. At a certain point in the combustion reaction, called the ignition point, there will be a flame. Fire is the visible part of fire. The flame is mainly composed of carbon dioxide, water vapor, oxygen and nitrogen. If the temperature is high enough, the gas may be ionized to produce a plasma. Depending on the substance being ignited and any foreign matter on the outside, the color and intensity of the flame will be different.

The most common form of fire can cause combustion and may cause physical damage through combustion. Fire is an important process that affects the global ecosystem. The positive effects of fires include stimulating growth and maintaining ecosystems.

The negative effects of fire include harm to life and property, air pollution and water pollution. If the fire clears the protective vegetation, heavy rain may lead to increased soil erosion. In addition, when burning plants, the nitrogen contained in them will be released into the atmosphere, which is different from the elements such as potassium and phosphorus that are left in the ashes and quickly recycled into the soil. The loss of nitrogen caused by fire will reduce soil fertility for a long time. When nitrogen is "fixed" in the atmosphere through lightning and legumes (such as clover), soil fertility will only recover slowly.

Humans have used fire in rituals, in agriculture for clearing land, cooking, generating heat and light, for signaling, propulsion purposes, smelting, forging, incinerating waste, cremating, and as weapons or means of destruction.