Plain-language answer: dBm is an absolute power unit expressed in decibels relative to one milliwatt.
Why the definition matters
It is convenient for receivers and signal chains because cable loss and gain add directly in decibels, but the physical power may be tiny.
Use the number correctly
Use 10 log10 of power in milliwatts, preserve the sign, and distinguish dBm power from dB ratio and dBµV voltage.
The common mistake
Dropping the m and turning an absolute level into an undefined ratio.
Measurement and safety boundary
Keep reference units, bandwidth, detector, direction, averaging, and test conditions attached to every number. Current regulator requirements and the complete installed system control RF-exposure compliance.
