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The watt (symbol: W) is a derived unit of power in the International System of Units (SI) defined as 1 joule per second and can be used to quantify the rate of energy transfer. Power has dimensions of ML2T-3.
The milliwatt (mW) is equal to one thousandth (10−3) of a watt. A typical laser pointer outputs about five milliwatts of light power, whereas a typical hearing aid for people uses less than one milliwatt. Audio signals and other electronic signal levels are often measured in dBm, referenced to one milliwatt.