  Multi-system interaction is an important and difficult problem in physics.
Motivated by the experimental result of an electronic circuit element
"Fractor", we introduce the concept of dynamic-order fractional dynamic system,
in which the differential-order of a fractional dynamic system is determined by
the output signal of another dynamic system. The new concept offers a
comprehensive explanation of physical mechanism of multi-system interaction.
The properties and potential applications of dynamic-order fractional dynamic
systems are further explored with the analysis of anomalous relaxation and
diffusion processes.
