The (\text{dBm}) is an absolute unit of power, where the reference power ((P_{ref})) is always 1 milliwatt ((\text{1 mW})). It is the standard unit for expressing transmitter output power and receiver sensitivity in both (\text{RF}) and optical power systems.
$$P (\text{dBm}) = 10 \cdot \log_{10} \left( \frac{P (\text{mW})}{1 \text{ mW}} \right)$$Conversion: You can convert directly between (\text{dBm}) and Watts:
$$\text{Power} (\text{mW}) = 10^{\frac{P (\text{dBm})}{10}}$$