What does combustible mean?
"Combustible" describes anything capable of catching fire and burning — paper, wood, gasoline vapors, dry leaves. It functions chiefly as an adjective ("a highly combustible mixture") but also as a noun in chemistry and industry, where a "combustible" is simply a substance used as fuel. The word carries a technical, formal register: it appears constantly in fire codes, insurance assessments, laboratory manuals, and shipping regulations, where distinguishing combustible from non-combustible materials can be a matter of life and death. Its Latin-derived root links it to "combustion," the very process it describes. Although "flammable" has largely taken over everyday usage — largely because "inflammable" misleadingly sounded like its own opposite — "combustible" remains indispensable wherever precision about fire risk matters. Knowing it means understanding not just a label but an entire vocabulary of safety, science, and engineering.
Capable of catching fire and burning.
"Never store combustible liquids near a heater."
Frequently qualified as 'highly combustible' or 'readily combustible' in technical writing.
A substance capable of burning, especially one used as fuel.
"Wood, coal, and oil are common combustibles."
Used mainly in chemistry, engineering, and industrial contexts; often appears in the plural ('combustibles').
As a countable noun referring to burnable substances, typically in technical or regulatory contexts.
"The crew removed all combustibles from the deck before welding began."
'Inflammable' sounds like it should mean fireproof — it doesn't, which is why safety labels switched to 'flammable' — but 'combustible' never had that trap: it has always meant exactly what it says.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of combustible
Combustible derives from the Latin verb "comburerе" (to burn up), via its past participle stem "combustus"; the related noun "combustio" gives us "combustion." The word entered English in the early modern period, likely in the late sixteenth or seventeenth century, alongside the growth of chemistry and natural philosophy. Its Romance-language cognates include French "combustible" and Spanish/Italian "combustibile," all tracing back to the same Latin root.
Related word forms
How combustible is actually used
Mostly technical or formal register, common in fire-safety regulations, chemistry, and legal contexts. In everyday speech, 'flammable' is now more frequent for describing things that catch fire, partly because of safety-labelling conventions; 'combustible' retains a slightly more scientific tone.
Easily confused with combustible
Inflammable means the same as flammable (easily set on fire) — the 'in-' is from Latin 'inflammarе' (to set aflame), not the negative prefix — whereas 'combustible' emphasizes capability of burning; both describe fire-prone materials.