Linea Nigra
Linea nigra (Latin for “black line”) is a dark vertical line that appears on the abdomen during pregnancy. The brownish streak is usually about a centimeter in width. The line runs vertically along the midline of the abdomen from the pubis to the xiphoid process — the bottom-most part of the rib cage in the center or tip of the sternum.
It is a type of hyperpigmentation resulting from increased production of the pigment melanin thought to be caused by increased estrogen, the same process that causes the areolas to darken. Why this process of hyperpigmentation occurs on the midline of the abdomen is uncertain. Fair-skinned women show this symptom less often than women with darker pigmentation. Before it appears it may be more faintly visible as a linea alba (”white line”).
Linea nigra tends to appear around the second trimester. Some research suggests there may be a link between linea nigra and insulin. Supplementing the diet with folic acid is said to reduce the discoloration. After birth, the line fades slowly, but may never disappear entirely, and sun exposure may cause it to recur.
My definition:
That line that just made me realize my belly button is NOT in the centre of my abdomen.