vernier

Etymology
Borrowed from. From Pierre Vernier, the inventor.

Noun

 * 1) A secondary scale with finer graduations than the primary scale of a measuring device; the vernier measures between graduations of the larger scale.
 * 2) A secondary control input with finer control than the primary, or coarse, input; for example the vernier frequency tuning knob on a radio.
 * 3)  A secondary system of force application for the attitude control of a spacecraft; for example a vernier thruster.

Etymology
.