Glossary:Frequency
Frequency refers to the rate at which something happens or is repeated. If a time series has a constant time interval between its observations, this interval determines the frequency of the time series (e.g. monthly, quarterly, yearly). Frequency is closely associated with "Periodicity", to form a single entity referred to as "frequency and periodicity".
While frequency pertains to the time interval between the observations of a time series, periodicity refers to the frequency of the compilation of data (e.g. a time series is available once a year, an annual frequency, whereas the underlying data are compiled monthly, a monthly periodicity).