万一 (wàn yī): just in case; in case

You'd better bring an umbrella, just in case it rains.

Bring some extra cash; just in case your phone runs out of battery, you can still pay with cash.

Check the doors and windows before leaving; just in case you forget to lock them, a thief might break in.

I wrote down the teacher's phone number, just in case there's an emergency and I need to contact her immediately.

Finish your homework before playing games; just in case the teacher suddenly checks tomorrow, you won't be afraid.