This is said to be an “idealized” portrait of Simonetta Vespucci, a renowned beauty of Florence, painted by Botticelli after her death in 1476.
I saw it recently at the exhibit on renaissance portraits at the Metropolitan Museum in NYC. It was one of two paintings, both female portraits, by Botticelli said to be Vespucci, but there was a decided difference in the facial features. Vespucci is also suggested as the model for Venus in Botticelli’s Birth of Venus (1485).
Vespucci was only 22 when she died, but already said to be the most beautiful woman in Florence, the ideal of Renaissance female perfection. Botticelli did not name Vespucci as his model–or ideal–but art historians generally agree on this portrait, at least, if not the Venus.