What is 0dbm?

[Terry Fox]

In playing around again, I am trying to figure out how many millivolts
peak-to-peak is 0dbm into 50 ohm load.  I have a chart from Andre that
indicates 0dbm should be .225V, but is that rms, peak, or peak-to-peak.
I did this last year, and convinced myself that it is rms, and 0dbm
should be about .636V p-p, but in testing with real-world equipment I am
now not so sure.  Can somebody take a relatively accurate 0dm LF/HF
signal source, put it into a known 50 ohm load, and read off the
peak-to-peak voltage ON A SCOPE for me?

I have done the above, but at a frequency where I don't trust the scope
(50 MHz).  In addition, my Advantest spectrum analyzer from work says
one thing, and an HP RF power meter (microwave) says something else.

Sorry to ask such a basic question, but it will help me calibrate
stuff.  I guess 0dbm isn't as important as some accurate level that I
can see on a scope and analyzer (+3, +10, +17 dbm).  I should know this,
but my head starts hurting whenever I try to figure it out.

Terry  (tfox@erols.com)