A power measurement which is referenced to 1 mW. While the ‘z’ (often omitted) is the impedance of the circuit measured. When z is omitted it is assumed to be 600 ohm. If 0 dBm is measured across a 600 ohm load (denoted most clearly as “dBm (600 ohm)”) we know that 0.001 W is consumed.
0 dBm = 1 mW +15 dBm = 31.6 mW
The dBm can be related to the dBV: dBm = dBV – 2.21