Receiver sensitivity parameter


A receiver's LW and MW sensitivity is often specified in mV/M.  What does this actually mean?  I have tried to find out on the web but have yet to find an answer that I can understand.

I presume it means millivolts per metre.  Millivolts I understand but to what does the metre refer?  Does it refer to the wavelength of the received signal?  Does it mean that a receiver, with a MW sensitivity of 1mV/M tuned to a frequency of 1Mhz, has a raw sensitivity of 300mV?  Or does it calculate in some other way?

Can anyone please enlighten me?

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