Is there a scientific definition for "impossible"?

Question

For example, if something has less than a .00001% chance of happening, it would be considered impossible. Is there any formula or percent chance that can define if something is impossible?

Answer 1

Yes typically 0.000001% chance or less is considered impossible ;-)

There is no formal definition as far as I know. Sometimes 0.01% or even 1% is considered impossible.

Answer 2

No. Impossible is not a scientific word. Improbable is the word that you are looking for.