In geometry, a 20-diagonal refers to the number of diagonals in a polygon with 20 sides (20-gon). The formula to calculate the number of diagonals in a polygon with ( n ) sides is given by ( \frac{n(n-3)}{2} ). For a 20-gon, this results in ( \frac{20(20-3)}{2} = \frac{20 \times 17}{2} = 170 ) diagonals. Thus, a 20-gon has 170 diagonals.