Definition of "vernier"

A secondary scale with finer graduations than the primary scale of a measuring device; the vernier measures between graduations of the larger scale.  examples

A secondary control input with finer control than the primary, or coarse, input; for example the vernier frequency tuning knob on a radio.  examples

(physics) A secondary system of force application for the attitude control of a spacecraft; for example a vernier thruster.  examples