(Originally) a device for counting and displaying the number of revolutions made by an engine or other mechanism; (in later use) one for displaying the rate at which a shaft or an engine is turning.
Origin
Mid 19th century.
Definition of revolution counter in US English:
revolution counter
noun
(Originally) a device for counting and displaying the number of revolutions made by an engine or other mechanism; (in later use) one for displaying the rate at which a shaft or an engine is turning.