Source : Webster's Revised Unabridged Dictionary (1913)

Cardioid \Car"di*oid\, n. [Gr. kardio-eidh`s heart-shaped;
   kardi`a heart + e'i^dos shape.] (Math.)
   An algebraic curve, so called from its resemblance to a
   heart.

Source : WordNet®

cardioid
     n : an epicycloid in which the rolling circle equals the fixed
         circle