polygonal number
nounUK /pəˈlɪɡənəl ˈnʌmbə/
US /pəˈlɪɡənəl ˈnʌmbər/
A polygonal number is a number you can make by arranging dots into a shape with straight sides.
Ten is a triangular polygonal number because you can arrange ten dots into a triangle.
OTHER MEANINGS
2. A number that counts the dots needed to build a regular polygon of a given size.
The fourth triangular number is ten.
3. Any number in a sequence where each term adds a growing number of dots to form a larger polygon.
Square numbers like 1, 4, 9, and 16 are polygonal numbers.
4. A number that can be shown as a regular arrangement of points forming a many-sided shape.
Twenty-eight is a hexagonal polygonal number.