The Collaborative International Dictionary of English v.0.48:

Alidade \Al"i*dade\, n. [LL. alidada, alhidada, fr. Ar.
   al-'id[=a]da a sort of rule: cf. F. alidade.]
   The portion of a graduated instrument, as a quadrant or
   astrolabe, carrying the sights or telescope, and showing the
   degrees cut off on the arc of the instrument --Whewell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alidade
    n 1: surveying instrument consisting of the upper movable part
         of a theodolite including the telescope and its attachments
         [syn: alidade, alidad]
    2: surveying instrument used with a plane table for drawing
       lines of sight on a distant object and for measuring angles
       [syn: alidade, alidad]