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Fire Behavior Glossary: Key Terms for Firefighters

Clear definitions of the words used to describe fire: flash point, flammable range, flashover, backdraft, heat transfer and more.

Lesson 8 of 8 · About 5 min read · Reviewed October 16, 2026

Key takeaways

Flash point, fire point and ignition temperature describe three different thresholds.
LEL and UEL bound the flammable range; too lean and too rich will not burn.
Flashover and backdraft are different events with different causes.
Use the same words your department uses, so everyone on scene shares one picture.

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Fire behavior terms and definitions

Autoignition (ignition) temperature: The minimum temperature to which a fuel in air must be heated to start self-sustained combustion without an outside flame or spark.
Backdraft: A rapid or explosive burning of hot, fuel-rich gases that occurs when air reaches a ventilation-limited compartment.
BLEVE: Boiling liquid expanding vapor explosion: the failure of a pressurized container holding a liquid above its normal boiling point, followed by rapid vaporization.
Boiling point: The temperature at which a liquid’s vapor pressure equals the pressure around it, so it boils.
British thermal unit (BTU): The amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit.
Calorie: The amount of heat needed to raise the temperature of one gram of water by one degree Celsius.
Celsius (Centigrade): A temperature scale on which water freezes at 0 degrees and boils at 100 degrees at sea level.
Chain reaction (chemical): The self-sustaining series of reactions in flaming combustion, carried by reactive fragments called free radicals. It is the fourth side of the fire tetrahedron.
Combustible liquid: A liquid with a flash point at or above 100°F (37.8°C).
Combustion: A self-sustaining chemical reaction in which a fuel is rapidly oxidized, giving off heat and usually light.
Conduction: Heat transfer through solids or between materials in direct contact.
Convection: Heat transfer by the movement of a gas or liquid.
Decay stage: The stage of a compartment fire when fuel or oxygen runs low and temperatures fall.
Endothermic reaction: A chemical reaction that absorbs heat energy.
Exothermic reaction: A chemical reaction that gives off heat energy.
Fahrenheit: A temperature scale on which water freezes at 32 degrees and boils at 212 degrees at sea level.
Fire gases: The gaseous products of combustion, including carbon monoxide, carbon dioxide and hydrogen cyanide.
Fire point: The temperature at which a liquid gives off enough vapor to keep burning once ignited. It is usually a few degrees above the flash point.
Fire tetrahedron: The model of flaming combustion with four sides: fuel, heat, oxidizer and uninhibited chemical chain reaction.
Fire triangle: The older model with three sides: fuel, heat and oxygen. It still describes smoldering combustion.
Flame: The visible, luminous part of a burning gas.
Flammable (explosive) range: The range of fuel vapor concentration in air that will burn when ignited, between the lower and upper limits.
Flammable liquid: A liquid with a flash point below 100°F (37.8°C).
Flash point: The lowest temperature at which a liquid gives off enough vapor to form an ignitable mixture with air near its surface. The vapor will flash but may not keep burning.
Flashover: The transition from a growing fire to a fully developed fire, when radiant heat causes exposed surfaces in a compartment to ignite at about the same time.
Flow path: The route that air and hot gases follow through a structure, from inlet to exhaust.
Free radical: A highly reactive molecular fragment that carries the chain reaction in flaming combustion.
Fuel-controlled fire: A fire with enough air that its growth is set by the amount and arrangement of the fuel.
Fully developed stage: The stage when all available fuel in a compartment is burning and heat release is at its peak.
Heat: Energy transferred because of a temperature difference. It is measured in joules, calories or BTUs.
Heat flux: The rate of heat transfer to a surface, usually measured in kilowatts per square meter.
Heat release rate (HRR): The rate at which a fire gives off energy.
Hydrogen cyanide (HCN): A toxic gas produced when nitrogen-containing materials burn. It blocks cells from using oxygen.
Lower explosive limit (LEL): The lowest concentration of a fuel vapor in air that will burn. Below it, the mixture is too lean.
Neutral plane: The level at an opening where outflowing hot gases meet inflowing air.
Oxidation: A chemical reaction in which a substance combines with oxygen or another oxidizing agent to form more stable compounds.
Oxidizer: A substance that gives up oxygen or other oxidizing gases in a reaction and supports combustion.
Pyrolysis: The chemical decomposition of a solid by heat, releasing flammable gases.
Radiation: Heat transfer by infrared waves, with no contact or medium needed.
Rollover: Flames moving through the hot gas layer near the ceiling, without the whole room igniting.
Smoke: A mixture of gases, solid soot particles and liquid droplets produced by burning.
Smoke explosion: Ignition of accumulated smoke and gases in a space away from the fire, such as a void or adjoining room.
Smoldering: Flameless combustion of a solid, seen as glowing with smoke.
Specific gravity: The density of a liquid compared with water (water = 1).
Spontaneous heating: Self-heating of a material with no outside heat source.
Upper explosive limit (UEL): The highest concentration of a fuel vapor in air that will burn. Above it, the mixture is too rich.
Vapor density: The weight of a gas or vapor compared with air (air = 1).
Vaporization: The change of a liquid into a vapor.
Ventilation-limited fire: A fire in which the oxygen supply, not the fuel, sets the heat release rate.
Volatility: How easily a liquid gives off vapor.

Fireground slang you will hear

The fire service has its own language. These are common terms; meanings can vary by department.

Black fire: Dense, dark, fuel-rich smoke with little visible flame. Hot and ready to ignite.
Knockdown: Reducing a fire’s intensity so it is no longer growing.
Overhaul: Finding and extinguishing hidden fire after the main fire is knocked down.
Rekindle: A fire that restarts after seeming to be out.
Pushing fire: Moving a fire to places you did not intend, for example with water or ventilation.
Transitional attack: Applying water from outside to the fire compartment before moving inside.

Fireground talk

Size-up: Gathering and weighing the conditions on arrival to decide a plan of attack.
Conditions: What the fire, smoke and building are doing right now.

Practice it in the simulators

Study questions

What is the difference between flash point and fire point?

Flash point is the lowest temperature at which a liquid gives off enough vapor to ignite briefly. Fire point is slightly higher: the temperature at which the vapor keeps burning once it is ignited.

What is the difference between LEL and UEL?

The lower explosive limit is the lowest vapor concentration in air that will burn. The upper explosive limit is the highest. Between them is the flammable range.

What is a BTU?

The amount of heat needed to raise one pound of water by one degree Fahrenheit.

What does exothermic mean?

A chemical reaction that gives off heat. Combustion is exothermic.

Sources and further reading

41 CFR 102-80.145: What is meant by flashover? — U.S. Code of Federal Regulations (Cornell LII)
OSHA Chemical Database: Propane — OSHA / NIOSH
NFPA 30, Flammable and Combustible Liquids Code — NFPA (print standard)
NFPA 921, Guide for Fire and Explosion Investigations — NFPA (print standard)
Essentials of Fire Fighting, IFSTA — Fire Protection Publications (print textbook)

About this guide. The Fire Behavior & Chemistry series was originally informed by LBFD Training Division recruit training material. This edition has been independently rewritten and expanded using the sources listed on each page, and its diagrams were drawn for this site. Free Fire Simulator is not affiliated with or endorsed by LBFD. Spotted an error or have a better source? Use the Contact Us button.