Heart

a shape that is not a heart

Unicode: U+2665 black heart suit · U+2764 heavy black heart
Meaning: Love, affection, the seat of feeling. The form bears no resemblance to the human organ, and never did.
History: The iconographic record begins in later medieval Europe: the shape appears in thirteenth- and fourteenth-century manuscripts as a lover’s heart, offered or held, in a decorative vocabulary already full of similar leaf forms. It became one of the four French playing-card suits in the fifteenth century, and the modern rounded version follows from that standardisation.
Variants: The suit glyph, the outline, and the heavy emoji form differ in weight and proportion; the two-lobed medieval version is narrower and more pointed than anything used today.
Related: ♠ ♦ ♣ the French suit system; ☮ another mark whose folk origin story outruns its documented one.
Appears: Playing cards, valentines, road signage in a few jurisdictions, the most-used emoji on earth, and — since Milton Glaser — a verb.
Evolution: Decorative leaf form → courtly love emblem → card suit → logo rebus → emoji → a word people say aloud.

Sources · 4

documented Milton Glaser, I ♥ NY, 1977 — the moment the glyph starts working as a rebus and then as a verb.
documented The French suit system, c. 1480, which fixed the shape that spread.
folk That the shape derives from the seedpod of silphium, the extinct Cyrenaican contraceptive plant shown on ancient coins. Endlessly repeated online. The coins are real and the pods are heart-shaped; what is missing is any evidentiary chain between them and love iconography appearing in Europe a thousand years and more later.
traditional That it depicts the heart as described in ancient and medieval anatomy. Plausible, occasionally argued, and not established.