1.
plain as day; obvious beyond doubt; patently obvious
So obvious that no one could doubt or deny it, like the unmistakable brightness of fire. Used to emphasize that something is self-evidently true.
A proverb-like expression meaning something is so obvious it cannot be doubted. The image comes from the unmistakable brightness of fire — just as you cannot mistake fire when you see it, the truth of the statement cannot be questioned.
Common patterns
Always used in the pattern 〜は火を見るより明らかだ, where the preceding clause states the obvious fact.
Etymology
Derived from classical Chinese (尚書). The original idea is that fire is the most unmistakably visible thing, so something "clearer than seeing fire" is absolutely certain.
Register
Somewhat formal and literary. Used in written language, speeches, and argumentative contexts. In casual conversation, people would more likely say 絶対に〜 or 間違いなく〜.