The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
cardioid n 1: an epicycloid in which the rolling circle equals the fixed circle