dBm0 is an abbreviation for the power in dBm

DBm

dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...

 measured at a zero transmission level point.

dBm0 is a concept used (amongst other areas) in audio/telephony processing since it allows a smooth integration of analog and digital chains. Notably, for A-law and μ-law codecs the standards define a sequence which has a 0 dBm0 output.

Note 1: A consequence for the A-law and μ-law codecs of the 0 dBm0 definition is that they have a respective 3.14 dBm0 and 3.17 dBm0 maximum signal level (ratio between the maximum obtainable sine wave amplitude and the specified reference 0 dBm0 sine wave amplitude).

Note 2: 0 dBm0 is often replaced by or used instead of digital milliwatt

Digital milliwatt

In digital telephony, the digital milliwatt is a standard test signal consisting of eight 8-bit words corresponding to one pulse-code modulated cycle of a sinusoidal signal approximately 1 kHz in frequency and one milliwatt in power....

 or zero transmission level point.