The term is infeasible. Then again just about anything which most people consider is impossible is simply low probability. Trying running directly into a wall. There is a non zero percent chance that due to quantum effect you will pass right through it.
Thanks, I was searching for it but I was coming up with a mental blank. I'm just really nitpicky about when people say something is completely impossible to do, when in fact there is just a ridiculously low probability (as to me there is a difference).