The expression dBm is used to define signal strength in wires and cables at RF and AF frequencies. The symbol is an abbreviation for "decibels relative to one milliwatt," where one milliwatt (1 mW) equals 1/1000 of a watt (0.001 W or 10 -3 W). This unit is commonly used in test laboratories.
S dBm = 10 log 10 P
A 1-mW signal has a level of 0 dBm. Signals weaker than 1 mW have negative dBm values; signals stronger than 1 mW have positive dBm values.