A plane figure with nine straight sides and nine angles.‘The room is shaped as a large nonagon, its sheer walls, a flat metallic grey, rising almost two stories up and curving into a dome.’
- ‘What's the perimeter of a nonagon whose sides are each twice the length of the side of a square whose area is 16 square units?’
Mid 17th century formed irregularly from Latin nonus ‘ninth’, on the pattern of words such as hexagon.
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