The word analog is derived from the Greek ana-logon, meaning "according to a ratio." A computer that represents numbers by some continuously variable physical quantity, whose variations mimic the properties of some system being modeled. Analogue computers have been built using mechanical motion (such as rotation), pneumatic or hydraulic pressure, or electrical voltage as the requisite quantity.
For example, an old fashioned carburetor may be considered a simple analogue computer that computes a petrol/air mixture strength function given the inputs of throttle pedal position and engine airflow.
The military has used analogue computers for artillery range finding, and they are used to simulate car suspensions and similar elastic systems where real-time performance is valuable. However, in common with other analogue circuits, they lack digital logic's robustness against errors introduced by stray fluctuations.
In an electronic analog computer, the output voltage of an operational amplifier varies in response to its input signals analogously to how a variable in the physical system being modeled responds to its conditions.
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