The half of a diameter of a circle is called a radius.
To elaborate, the diameter is the longest distance across a circle, passing through the center and connecting two points on its circumference. When you take half of that distance, you end up with the radius, which extends from the center of the circle to any point on its edge. Essentially, while the diameter gives you the full width of the circle, the radius tells you how far the center is from the outer edge, which is a crucial measure in various geometrical calculations and applications.