i understand that, periodic behavior is defined as recurring at regular intervals. Does quasiperiodic mean, that the interval is shifted. If yes, is this shift well defined?
Yes, quasiperiodic means that the period shifts, and no the shift is not always well defined. It can be, for example, in the effect of general relativity on the orbit of the planet mercury. For many situations associated with nonlinear systems, the shifts from exact periodicity are not so regular.