The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Vernier
    n 1: French mathematician who described the vernier scale
         (1580-1637) [syn: Vernier, Paul Vernier]
    2: 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, vernier]