noun
fuse; pilot light
Literal use: a small flame that ignites a larger fire, such as a pilot light in a gas appliance or a fuse for fireworks.
ガスコンロの口火が消えていた。
The pilot light on the gas stove had gone out.
花火の口火に火をつける。
Light the fuse of the firework.
noun
Figurative use: the initial event or remark that sets off a larger conflict, argument, or movement. Often used in the phrase 口火を切る (to start something, to break the ice).
その小さな衝突が戦争の口火となった。
That small clash became the trigger for the war.
彼の一言が議論の口火を切った。
His remark sparked the debate.