Plural of alidade; a surveying instrument consisting of a ruler with sights used for measuring angles and distances on maps.
From Arabic al-'idhādah meaning 'the revolving radius,' brought into European scientific terminology through medieval Arabic mathematical texts.
Before GPS, surveyors used alidads to map the entire world—it's basically a sighting ruler that rotates, and many historical maps we still use today were created by people carefully aligning these simple but brilliant instruments.
Complete word intelligence in one call. Free tier — 50 lookups/day.