dB (decibel) is a unit that describes the ratio between two power levels, such as the amount of attenuation or gain in a signal. It is a logarithmic unit that expresses the power level as a ratio of the power of the reference level. dBm (decibel-milliwatt) is a unit used to express an absolute power level, referencing 1 milliwatt (mW). It is used to measure the actual power of an optical signal rather than the ratio of power levels.