Indeed, 0dBm is 1 milliwatt, a measure of power, so it is 1mW into
whatever your system impedance happens to be, the actual voltage (&
current) will depend on your system impedance, 50, 75, 600 ohms or
whatever.

Quite. However, it's unfortunate that the 'W' seems to have got dropped
off the end - thus allowing dBm to be used carelessly and incorrectly
(as Jerry seems to have been doing).

But it always has been so, and I don't think it is a case of the 'W'
being forgotten, it was never there in the first place since the 40's
when dBm started to come into use.

Jeff