The diameter of a circle is a straight line that passes through its center and touches the circle at two points on opposite sides. It is also the length of that line, and is twice the radius.
From Greek "diametros" meaning "measure across," formed from "dia-" (across) and "metron" (measure). It entered English through Latin and Old French mathematical language.
Diameter is literally the "across-measure" of a circle. Once you know the diameter, you can unlock a lot of other facts about the circle—like its circumference—because π quietly links them all together.
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