The National Cancer Institute defines 
"Aesthetic Flat Closure"

(aka 'going flat')

as:

A type of surgery that is done to rebuild the shape of the chest wall
after one or both breasts are removed. 

An aesthetic flat closure may also be done after removal of
a breast implant that was used to restore breast shape.

During an aesthetic flat closure, extra skin, fat, and
other tissue in the breast area are removed. 

The remaining tissue is then tightened and smoothed out
so that the chest wall appears
FLAT.