mechanically-adjustable special-type variable resistor with 3 legs and it is used as a potential-divider. (It is not a meter at-all, but got the name from its historical ancestor which was a meter.)
A potentiometer, (also known as pot, control, volume-control, Volume-knobs, etc). It is a variable, voltage-divider (potential divider).
A Potentiometer contains a resistance wire (wound on a former) or a carbon-film, whose both-termini are connected with separate leads (end-terminal or terminal-legs). On this resistance-wire, there is a slider (or also called wiping-contact, or variable-contact, or jockey etc). This slider is made up of conductive-substance, and connected to the variable-contact terminal (usually the central-terminal or central-leg).
Conceptually, potentiometers are different from rheostats, because rheostats (the basic-type of variable resistor) contains only 2 terminus. (What's the difference between a potentiometer and a rheostat?) But, usually from market, separate rheoststs are not bought ... a potentiometer is used as rheostat by using only 1 of the terminal-leg, and the variable contact terminal.
Potentiometers can be of different types, such as sliding-type (Linear-type, or also called fader), rotary-type (commonly called volume-control or volume knobs; often also contains an on-off switch attached), and Preset-type potentiometers (also called trimmer-type potentiometers or trimming potentiometers or trim-pots).
"Potentiometer" got its name from its historic ancestor, a measuring-device working on the principle of wheatstone-bridge voltage divider. (The potentiometer used in electronics is not a measuring instrument. Why it is called a meter?)