An amplifier is a device for increasing the power of a signal by use of an external energy source.
In an electronic amplifier, the input "signal" is usually a voltage or a current. Other types exist; a fluidic amplifier increases the power of signals represented as flow of gas or liquid, for example. Amplifiers may be classified in a variety of ways depending on their application, the frequency range they cover, or the active devices used. Ideally an amplifier increases the power of a signal without otherwise altering it; practical amplifiers have finite distortion and noise which they invariably add to the signal.
A device that converts signals from one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor. However, none of these amplify power.
Read more about Amplifier: Figures of Merit, Electronic Amplifiers