A chord of a circle $O$ is a line segment joining two points on $O$.

[asy]size(100); pair O=origin,A=dir(135),B=dir(30); D(unitcircle); D(A--B); MP("O",D(O),S); MP("A",D(A),W); MP("B",D(B),E);[/asy]

The diameter of a circle is the longest chord of that circle. The diameter goes through the center of the circle.

[asy]size(120); pair O=origin,A=dir(170),B=dir(-10); D(unitcircle); D(A--B); MP("O",D(O),N); MP("A",D(A),W); MP("B",D(B),E);[/asy]

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