In a series of observations, or trials, the relative frequency of occurrence of an event E is calculated as the number of times the event E happened over the total number of observations made. The relative frequency density of occurrence of an event is the relative frequency of E divided by the size of the bin used to classify E.
The limiting relative frequency of an event over a long series of trials is the conceptual foundation of the frequency interpretation of probability. In this framework, it is assumed that as the length of the series increases without bound, the fraction of the experiments in which we observe the event will stabilize. This interpretation is often contrasted with Bayesian probability.