Neckline

[Sarah]

the shape made by the edge of a dress or shirt at the front of the neck or on the chest:

[Marquetta]

The edge of a womans garment at or below the neck, used with reference to its height or shape.

[MashaDit]

the line formed by the edge of a garment around the neck

[dipangkarbos]

The neckline is the top edge of a garment that surrounds the neck, especially from the front view. Neckline also refers to the overall line between all the layers of clothing and the neck and shoulders of a person, ignoring the unseen undergarments.
For each garment worn above the waist, the neckline is primarily a style line and may be a boundary for further shaping of the upper edge of a garment with, for example, a collar, cowl, darts or pleats. In that respect it is similar to the waistline and hemline.