What does alidade mean?
The definition of alidade
Al”i*dade, n.
Etym: [LL. alidada, alhidada, fr. Ar. al-‘idada 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.