Polymastia

[Merna]

[Accessory] breasts, also known as polymastia, [supernumerary] breasts, or mammae erraticae, is the condition of having an additional breast. Extra breasts may appear with or without nipples or [areolae].
A related condition, in which extra nipples form, is called supernumerary nipple or polythelia.

[Julene]

The congenital condition of having supernumerary breasts.