Dart
A folded and stitched wedge of fabric that takes in the extra fabric so a flat piece fits the curve of the body.
Darts give shape at the bust, waist, hip, back shoulder and other places where the body curves. The wide end of the dart sits at the edge of the piece and its point aims at the fullest part of the curve. In a pattern, darts can be moved, split or turned into seams or gathers without changing the fit, which is a basic technique of flat-pattern manipulation.
Example
On a bodice with a waist dart, the difference between the bust and waist circumference is taken in by the dart, so the garment follows the waist instead of hanging straight.