Brief explanation what an EM mode is?

[dayan83]

Can anyone provide a brief explanation what an EM mode is? I've read some EM theory, but still have problems understanding what a mode really is! I know for some EM waves, the medium supports only certain modes, but is the 'mode' a property of a medium? Or is it something that should be understood with respect to the wave itself?

 

[Born2bwire]

A mode is an eigenstate of the system. For example, a rectangular cavity enclosed by conducting walls will only support specific resonant frequencies. The resonant fields at each of these frequencies are the modes. Fields that are excited at frequencies will not be supported and will die out unless continual power is inputted. If you excite at a resonant frequency, you usually do not need to excite the actual field distribution of the mode, usually the waves will propagate in a superposition of modes or will redistribute themselves into a supported mode. Also note that you may have more than one mode at a given frequency. These modes are properties of frequency, geometry, and the make-up of the background media.