gauge

An instrument that displays a measurement (of pressure or temperature, for instance) by moving a needle across a scale.

Examples of these can be found on post #1483841 (strapped to the girl's thigh) or post #1665641 (on the front of the aircraft). These sometimes take the form of a thermometer-like measurement bar, or a dial.

For video game interfaces that indicate energy, health, progress, etc., see health bar.

For the wide-diameter piercings colloquially called "gauges", see plug (piercing).