Periodicity is the quality of occurring at regular intervals or periods (in time or space) and can occur in different contexts:
The measure of periodicity in time is frequency which has the Metric units of Hertz.

A clock marks time at periodic intervals.
A metronome ticks at periodic intervals of time.
A publication published at periodic intervals can be called a "periodical", for example a magazine.
In mathematics, a function whose output contains values that repeat periodically is called a periodic function.
In chemistry, the periodic table is a table which classifies the chemical elements by means of the periodicity of their chemical properties.
In physics, period is the number of cycles as a result of time (time/cycle). The amount of time it takes to complete one full revolution. Period is also the inverse of frequency.
In music theory, periodicity is described as "predictability gives rise to expectations".
Standing waves crest at periodic intervals of distance.
In finance, the periodicity of a loan describes the interval between payments. Periodic Table
When used in the periodic table, periodicity is the way in which properties of elements are repeated, depending on their location in the periodic table.