A heart-shaped mathematical curve traced by a point on the circumference of a circle rolling around a fixed circle of equal size.
From Greek 'kardia' (heart) + '-oid' (resembling). Named in mathematics in the 1680s because its shape resembles a human heart, though mathematicians studied it before recognizing this similarity.
The cardioid appears everywhere in nature—in the ripples of coffee when a droplet lands, in certain microphone pickup patterns, and in the paths of planets—yet it's named after the human heart, not the other way around!
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