The amount of power out versus the amount of power you put in. Ideally, everything is 100% efficient, but nothing ever is. The closer you get, the better. You can usually get a rough estimate of how inefficient something is by the amount of heat it produces. If your phone gets hot when it’s charging, all that heat is evidence of lost power and reduced efficiency.
The ratio of the signal power delivered to a load to the power from the power supply of an amplifier.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
Ratio of power output to power input X 100%
Grob’s Basic Electronics, 11th Edition by Mitchel E. Schultz