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.
The pilot light on the gas stove had gone out.
Light the fuse of the firework.
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).
That small clash became the trigger for the war.
His remark sparked the debate.
I lit the pilot light of the gas stove.
His remark became the spark for a heated debate.
きっかけ is a neutral, everyday word for any opportunity or chance to start something, while 口火 is more dramatic and implies triggering a larger reaction such as an argument or movement.
口火 (くちび) is a fuse or figurative trigger, while 火口 (かこう) is a volcanic crater or the muzzle of a firearm; the two are easily confused because they share the same kanji in reverse order.
Compound of 口 (mouth, opening) and 火 (fire), literally 'mouth fire'. Originally referred to the small flame at the mouth of a firearm or a pilot light, and later extended figuratively to any initiating spark.