A potentiometer, informally a pot, is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
The measuring instrument called a potentiometer is essentially a voltage divider used for measuring electric potential; the component is an implementation of the same principle, hence its name.
Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment.
Potentiometers are rarely used to directly control significant power, since the power dissipated in the potentiometer would be comparable to the power in the controlled load.