The definition of a sweetheart neckline is a heart-shaped cut on the neck of a dress or blouse.
The heart-shaped neckline on a wedding dress is an example of a sweetheart neckline.
a neckline on a woman's or girl's garment with the front cut low and shaped to resemble the top half of a conventionalized heart
(plural sweetheart necklines)
From the heart shape.