A quasi-periodic phenomenon is one that almost has a periodic behaviour, but is not entirely predictable. They may occur when otherwise periodic processes have noise. For example, imagine a pedulum clock in a very windy location, the pendulum naturally swings once a secomd, but the gusts and breezes mean it may drift, with some swings a little faster and some a little slower. Critically, if two such pendula were in different environments and started at the same moment, after a while the pendula would both still be approximately once a second, but may be out of phase with one another.
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