In terms of probability, a(n) is any process with uncertain results that can be repeated.

In this problem, the answer would be probability experiment or just experiment or trial. Either of the three would be correct.

Through these experiments, people can determine which would cause which and which would result to which. More or less, the probability would be tested over and over again. After a quite number of experiments done, depending on the number of times it was done, it would then generate the empirical probabilities and then apply statistical analysis to it. Thus, in terms of probability, an experiment or a trial is any process with uncertain results that can be repeated.