A sighting ruler or straight edge with a pointer used on surveying instruments to measure angles and directions on maps and land surveys.
From Arabic 'al-idadah' (the revolving radius), adapted into English and other European languages, particularly through Spanish and Portuguese medieval and Renaissance sources.
Surveyors have used this device for 1,000 years, and every map you've ever seen was partially drawn with help from an alidade—it's an invisible tool that shaped the world's knowledge of geography.
Complete word intelligence in one call. Free tier — 50 lookups/day.