Based on the mid-sixteenth-century biography of Leonardo da Vinci by Giorgio Vasari, many historians believe the painting is a portrait of Madam Lisa Giocondo, wife of a wealthy Florentine. It is from Vasari that the painting received the name Mona Lisa, also known as La Gioconda in Italian or La Joconde in French. But Vasari published his book thirty-one years after Leonardo's death, and he was known to fill in fact with fragments of fantasy. Before Vasari, the painting had been referred to as "a certain Florentine lady" and later, in the collection at Fontainebleau, as "a courtesan in a gauze veil." There are many other theories about who the sitter might have been, based on bits of scattered evidence. But the panel is unsigned and undated, and although most portraits of the time included something to indicate the sitter's family name or social status, no such emblem can be found in the Mona Lisa.

Source: PBS.com