Periodicity and quasiperiodicity

Periodicty is the quality or state of being periodic, recurring at regular interval.

Periodic function
In mathematics a periodic function is a function that repeats its values in regular intervals. Examples are sinus, cosinus and tanget functions.

In physics periodic functions describe phenomena such as oscillations and waves.

Quasiperiodicity
Some systems display a pattern of recurrence with a component of unpredictability. The term quasiperiodicity can be used to describe such a system.

Physiology and medicine
In physiology/medicine the healthy heart rhythm (sinus rhythm), as measured by an ECG, is characterized by periodicity.

Heart rhythm disorders can be characterized by quasiperiodicity.