Source : Webster's Revised Unabridged Dictionary (1913)

Vernier \Ver"ni*er\, n. [So named after the inventor, Pierre
   Vernier.]
   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.

   {Vernier calipers}, {Vernier gauge}, a gauge with a graduated
      bar and a sliding jaw bearing a vernier, used for accurate
      measurements.

   {Vernier compass}, a surveyor's compass with a vernier for
      the accurate adjustment of the zero point in accordance
      with magnetic variation.

   {Vernier transit}, a surveyor's transit instrument with a
      vernier compass.

Source : WordNet®

vernier
     n 1: a small movable scale that slides along a main scale; the
          small scale is calibrated to indicate fractional
          divisions of the main scale [syn: {vernier scale}]
     2: French mathematician who described the vernier scale
        (1580-1637) [syn: {Paul Vernier}]