Frequency probability or Frequentism is one of the interpretations of probability theory. Repeating a scientific experiment very often gives a number of results. It is then possible, to count the number of times that a given event happened and compare it to the total number of experiments.
This interpretation of probabiilty was very important for statistics. People who use this interpretation are often called Frequentists. Well-known frequentists include Richard von Mises, Egon Pearson, Jerzy Neyman, R. A. Fisher and John Venn.