1. Noun

   1.

   (unlikely event of) emergency; the worst(-case scenario); 10000 to 1

2. Adverb (fukushi), Noun which may take the genitive case particle 'no'

   2.

   (if) by some chance; by some possibility; in the unlikely event that

Example sentences

• 万一病気になったらどうする？

  What would you do, if you should be taken ill?

• このお金は万一の時のためだよ。

  This money is for a rainy day.