From my personal view: wave is a disturbance travelling through space (not media). But there is a time delay dt which is propotional to distance between two points dx.
dx= v*dt; where v is the speed of the wave.
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For water wave, it is the disturbance of water. For sound wave, it is the disturbance of air.
For light or EM wave, it is a disturbance of both electric field and magnetic field.
The velue of speed of light in vacuum will be another value if we change the definition of "meter" and "second".
It is a result from experimental measurement.