diameter

nouns

the length of a straight line passing through the center of a circle and connecting two points on the circumference
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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)
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is a type of (narrower)
a line traced by a point traveling in a constant direction; a line of zero curvature
"the shortest distance between two points is a straight line"