Technically, the term is "zero probability". The classic mathematical example is throwing a dart with a tip of negligible width at a dartboard and expecting to hit a single mathematical point on the plane of that board. The point exists - it's defined - and therefore it is possible to hit it. However, there are so many (an infinite number) of other points that could be hit that, even given a well-aimed throw such that the chances of hitting a point on the dartboard is 1/1, the chances of hitting that exact point is 1/∞ = 0. Conversely, not hitting that single point is said to be "almost certain" in mathematical parlance.
Other terms that may fit:
practically impossible - this term allows for the technical possibility while still expressing the real-world impossibility.
infeasible - this is roughly synonymous with "improbable" or "unlikely", but has the connotation of "not doable", as in the action itself may be possible to accomplish, but the requirements involved behind the scenes to do it are prohibitive.
implausible - Again, technically possible, but the circumstances defy credibility.
However, remember the law of expected values; for any N, 1/N * N = 1. An infinitesimal chance, given infinite trials, is almost certain to happen once (there is always a mathematical chance that it will never happen on any N trials, but this chance approaches zero as N grows very large).