In the mathematics of probability, an impossible event is an event A with probability zero, or Pr(A) = 0.[1] See in particular almost surely.
An impossible event is not the same as the stronger concept of logical impossibility. For any continuous probability distribution the probability of any single elementary event is 0, yet the event is not logically impossible as an event outside the distribution. For instance, the probability of hitting any specific point on a dart board, let's say a square in Cartesian coordinates [−10, 10] × [−10, 10] and the point (4.5678, −8.4568), is 0, because there is an uncountably infinite number of points on the board. In contrast, hitting a point outside of the space considered is logically impossible.