A substance or agent that extinguishes fire or other things; a fire-suppressing chemical or material.
From 'extinguish' + '-ant' (agent or substance that performs an action). This technical term emerged in the 19th century with development of modern fire suppression chemicals.
When scientists invented new fire-fighting chemicals like halon or foam, they needed a word for the substance itself—'extinguishant' is that technical term, though firefighters usually just say 'suppressant'!
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