What probability is considered to mean "impossible". I have seen that 1/10e30 can mean impossible. So how many zeroes do you need ? Thanks.?

Impossible means a probability of 0.

Any probability greater than zero means the event is possible no matter how unlikely it might be.

A probability of 10^(-30) means it is unlikely the event will ever occur in any meaningful time span, but it is still possible.

In fact, there is no probability that is equivalent to impossibility. An impossible event has probability zero, but even zero probability does not mean the event is impossible.

Let random variable X have uniform distribution on [0, 1], a continuous interval.

For any x | 0 ≤ x ≤ 1, P(X = x) = 0.

To say that the event (X = x) is impossible would be to say that there are no numbers on the interval.

Your question appears to be one of applied statistics, not pure mathematics. I have seen statistical conclusions stating that an event is impossible. That would depend on standards laid out by the statistician. I really wish they would not use the word at all. In pure mathematics there is a world of difference between impossible and highly unlikely.

0 means it will never happen (impossible)

1 means it will always happen (certainty)

Any other value in between is possible even if it is infinitesimally small. For example, it is *possible* that someone could flip heads 100 times in a row, even if that is incredibly unlikely.