flashdamp
A mixture of methane, carbon dioxide, and nitrogen that is found in mines, and ignited quickly within a safety lamp, but that is then extinguished quickly due to the exhaust products of the flame. Flashdamp can be heavier than air (called heavy flashdamp) or lighter than air (called light flashdamp), depending on the proportion of carbon dioxide.
uncountable
Flashdamp is a more dangerous mixture than firedamp: first, because it is difficult of detection and often escapes the notice of the fire boss, who mistakes it for blackdamp, and has various ways of accounting for its position at the roof; second, because of the popular fallacy that this mixture, which extinguishes the flame of a lamp, is not explosive, whereas at certain stages of dilution it becomes highly explosive.
Where it comes from
From flash + damp, with 'damp' having the meaning of 'gas', as in old German, modern Dutch, and Danish.
Adapted from the Wiktionary entry flashdamp (CC BY-SA 4.0).