An Eigenmode (Normal Mode) of an oscillating system is
a mode where the whole system oscillates at the same
Frequency, i.e., resonating.
Calculating of eigenmodes is of use in many mechanical
oscillating systems, e.g., acoustics, seismic activity,
etc., as well as electrical systems and Quantum Mechanics calculations.
Eigenmode Decomposition or Eigenmode Expansion
can sometimes be done mathematically
and is used in simulation of Electromagnetic Radiation and in
Adaptive Optics and Back End Instruments.