Gauge (instrument)

A gauge, in science and engineering, is a device used to make measurements or in order to display certain dimensional information. A wide variety of tools exist which serve such functions, ranging from simple pieces of material against which sizes can be measured to complex pieces of machinery. Depending on usage, a gauge can be described as "a device for measuring a physical quantity", for example "to determine thickness, gap in space, diameter of materials, or pressure of flow", or "a device that displays the measurement of a monitored system by the use of a needle or pointer that moves along a calibrated scale".
A general and popular contention is that an instrument measures the absolute value of the metric as against a gauge which establishes and is concerned with the metric's compatibility with the specified end limits without giving exact value of the metric within these limits. 
Basic types.
All gauges can be divided into four main types, independent of their actual use.
The two first basic types are usually easier for the human eyes and brain to interpret, especially if many instrument meters must be read simultaneously. The other two types are only displaying digits, which are more complex for humans to read and interpret. The ultimate example is cockpit instrumentation in aircraft. The flight instruments cannot display figures only, hence even in the most modern "glass-cockpits" where almost all instruments are displayed at screens, few figures are visible. Instead the screens display analogue meters.
More in detail.
Various types of gauges include: