dBm (sometimes dBmW) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). 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. Compare dBW, which is referenced to one watt (1000 mW).
Since it is referenced to the watt, it is an absolute unit, used when measuring absolute power. By comparison, the decibel (dB) is adimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio.
In audio and telephony, dBm is typically referenced relative to a 600 ohm impedance, while in radiofrequency work dBm is typically referenced relative to a 50 ohm impedance.