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Half a line through a circle is called a "diameter" if it passes through the center of the circle, dividing it into two equal halves. If it does not pass through the center, it is referred to as a "chord." A diameter is the longest chord of the circle, while any chord that does not pass through the center is simply a shorter segment of the circle.
semicircleA half circle is called a semicircle.Half a circle is a semi-circle
To know what a radius is you have to kno what a diameter is. The diameter is the distance accross a circle through its center. The radius is half the diameter.
Typical answer from The WikiAnswers Community: IT IS WRONG!The diameter is the distance across the circle, through its centre. It is double the radius, not the other way round!