if by some chance; in the unlikely event
Used as an adverb to express a very low probability; often paired with conditional forms like ～たら, ～ば, or ～ても. The literal sense 'one in ten thousand' is rarely used in modern Japanese.
See also: 万一
If by any chance you think you'll be late, please call.
万が一のことがあっても、慌てないでください。
Even if the worst should happen, please don't panic.
emergency; worst-case scenario
Used as a noun, often in the phrase 万が一に備える (prepare for the worst). Implies an extremely unlikely but serious situation.
See also: 万一
万が一に備えて、保険に入っておいたほうがいい。
You should get insurance just in case (to prepare for the worst).
If there is ever an accident, please contact me immediately.
I keep emergency supplies ready to prepare for the worst.
Even if you happen to fail, please don't give up.
In case of an emergency, please call this number.