Half of a diameter is called the radius of a circle.

Quick Scoop: What does “half of diameter” mean?

  • In any circle, the diameter is the line passing through the center, touching the circle at two opposite points.
  • The radius is the line from the center of the circle to any point on the circle.
  • Because the diameter goes from one side of the circle to the other through the center, it is exactly twice the radius, so radius=diameter2\text{radius}=\frac{\text{diameter}}{2}radius=2diameter​.

So if a circle’s diameter is 10 cm, its radius (half of the diameter) is 5 cm.

In math questions, puzzles, and even crosswords, when they say “half of a diameter,” the expected single-word answer is radius.

TL;DR: Half of diameter is called the radius.

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