The Collaborative International Dictionary of English v.0.48:

Theodolite \The*od"o*lite\, n. [Probably a corruption of the
   alidade. See Alidade.]
   An instrument used, especially in trigonometrical surveying,
   for the accurate measurement of horizontal angles, and also
   usually of vertical angles. It is variously constructed.
   [1913 Webster]

   Note: The theodolite consists principally of a telescope,
         with cross wires in the focus of its object glass,
         clamped in Y's attached to a frame that is mounted so
         as to turn both on vertical and horizontal axes, the
         former carrying a vernier plate on a horizontal
         graduated plate or circle for azimuthal angles, and the
         latter a vertical graduated arc or semicircle for
         altitudes. The whole is furnished with levels and
         adjusting screws and mounted on a tripod.
         [1913 Webster]

WordNet (r) 3.0 (2006):

theodolite
    n 1: a surveying instrument for measuring horizontal and
         vertical angles, consisting of a small telescope mounted on
         a tripod [syn: theodolite, transit]