万一【wànyī】

万一 is literally "one in ten thousand". It brings up something with a small chance of happening, usually something you'd rather didn't. The most common frame is a worried question with 怎么办.

Just in case

After a precaution, 万一 + clause + 呢 gives the reason for it: "in case".

以防万一 is a set phrase for "just in case".

万一 or 如果

如果 is any "if". 万一 is only for an unlikely chance, so it doesn't fit an ordinary condition or a likely one.

• × 万一你有时间，来我家玩 → 如果你有时间，来我家玩

It can still set up a condition with 就 or 的话 in the main clause, as long as the chance is slim.