Power is the rate at which energy is supplied or consumed, often measured in Watts. This tag covers electrical power specifically, the voltage and current specifications for supplying power to the Pi, the specifics of power consumption by the Pi, and methods for controlling Pi power consumption.
From the Wikipedia article:
Electric power, like mechanical power, is the rate of doing work (W), measured in watts, and represented by the letter P. The electric power in watts produced by an electric current I consisting of a charge of Q coulombs every t seconds passing through an electric potential (voltage) difference of V is:
Q is electric charge in coulombs
t is time in seconds
I is electric current in amperes
V is electric potential or voltage in volts