vernier

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small, movable auxiliary graduated scale attached parallel to a main graduated scale, calibrated to indicate fractional parts of the subdivisions of the larger scale, and used on certain precision instruments to increase accuracy in measurement.
• noun An auxiliary device designed to facilitate fine adjustments or measurements on precision instruments.
• adjective Of, relating to, or having a vernier.

from The Century Dictionary.

• noun A small movable scale, running parallel with the fixed scale of a sextant. theodolite, barometer, or other graduated instrument, and used for measuring a fractional part of one of the equal divisions on the graduated fixed scale or arc.

from the GNU version of the Collaborative International Dictionary of English.

• noun A short scale made to slide along the divisions of a graduated instrument, as the limb of a sextant, or the scale of a barometer, for indicating parts of divisions. It is so graduated that a certain convenient number of its divisions are just equal to a certain number, either one less or one more, of the divisions of the instrument, so that parts of a division are determined by observing what line on the vernier coincides with a line on the instrument.
• noun a gauge with a graduated bar and a sliding jaw bearing a vernier, used for accurate measurements.
• noun a surveyor's compass with a vernier for the accurate adjustment of the zero point in accordance with magnetic variation.
• noun a surveyor's transit instrument with a vernier compass.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a small movable scale that slides along a main scale; the small scale is calibrated to indicate fractional divisions of the main scale
• noun French mathematician who described the vernier scale (1580-1637)