万が一

万が一 is a cautious adverb used to talk about a very unlikely or unexpected possibility. It means “in the unlikely event” or “just in case,” and is often followed by a clause describing what you would do or what might happen if that rare situation occurred. It can stand at the beginning of a sentence to set up a hypothetical condition, or be used with 万が一のことがあっても to emphasize that you will still proceed despite a rare risk. It’s common in cautious planning, warnings, or plans for contingencies.