RPM means "re-punched mintmark". There were many instances of mint marks being punched into the die in the wrong position. They were then repunched into (presumably) the right place and traces of the first mint mark can still be seen in the finished coin.
PMD means "post mint damage". Any kind of damage that happens after the coin leaves the mint is
PMD. I don't know about the mintmark on your nickel but the misshapen letters in In God We Trust are probably
PMD.