The phrase "flat nipples" refers to nipples that do not protrude outward when they become erect. Instead, they may remain flat or level with the surrounding skin. Full definition
You may be wondering can you breastfeed with flat nipples, and while the answer is generally a resounding «yes,» there are cases when it may be more difficult than others. (romper.com)
Flat nipples do not stick outward from the breast, but they don't turn inward (inverted nipples) either. (verywellfamily.com)
I had never heard of flat nipples but I was suddenly very ashamed of my body. (thebadassbreastfeeder.com)