Empirical means by observation, so empirical probability, or
experimental probability, is the probability that is observed in a
set of trials. For example, if you flip a coin ten times and get
seven heads, your empirical probability is 7 in 10. This is
different than the theoretical probability, which for a fair coin
is 5 in 10, but that result will only be approximated by the
empirical results, and then only with a larger number of
trials.