dBm is defined as power ratio in decibel (dB) referenced to one milliwatt (mW).
It is an abbreviation for dB with respect to 1 mW and the "m" in dBm stands for milliwatt.
dBm is different from dB.
dBm represents absolute power, whereas dB is a ratio of two values and is used to represent gain or attenuation. For example, 3 dBm means 2 mW, and 3 dB means a gain of 2.Similarly, -3 dBm means 0.5 mW, whereas -3 dB means attenuation of 2.