what is the meaning of periodicity

Periodicity is the quality of occurring at regular intervals (e.g. of time) and can occur in different contexts:

A clock marks time at periodic intervals.
A metronome ticks at periodic intervals.
A publication published at periodic intervals can be called a "periodical", though it can also be called magazine.
In mathematics, a function whose output contains values that repeat periodically is called periodic function.
In chemistry, a table which classifies elements by means of periodicity is the periodic table.
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".
The measure of periodicity is frequency.

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