A unit of measure used to reference signal strength level. unlike dB, this is relative measure, dBm is an absolute measure.
dBm ( Decibel-milliwatts ) or dBmW is a short from for the power ratio in decibels (dB) of the measured power referenced to one milli watt (mW). Most communication systems human speech, 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. The baseline reference relationship is 1mW = 0 dBm. A doubling of signal power yields an increase of approximately 3 dBm eg: (2mW = 3 dBm).