diameter diameter /daɪˈæmətɝ/

noun

1. () The length of a straight line passing through the center of a circle and connecting two points on the circumference  ( diam , diameter )
   the length of a straight line passing through the center of a circle and connecting two points on the circumference
    The diameter of the circle was 12cm
   The diameter of the circle was 12cm
   
2. () A straight line connecting the center of a circle with two points on its perimeter (or the center of a sphere with two points on its surface)  ( diameter )
   a straight line connecting the center of a circle with two points on its perimeter (or the center of a sphere with two points on its surface)
     The astronomer measured the diameter of the planet, determining its size.
   The astronomer measured the diameter of the planet, determining its size.