"Keyhole detail" refers to a small and usually decorative opening or shape in a piece of clothing, furniture, or other object, resembling the shape of a keyhole. Full definition
Round neckline Keyhole detail at back neck with self fabric covered button 65 % Rayon 30 % Nylon 5 % Spandex Machine wash (mayarya.com)
The round neck with keyhole detail shows off your cleavage while pleats neatly hide a tummy! (amazon.ca)
A little keyhole detail in the back gives it an extra touch and the fact that you can wear a regular bra with it makes it an absolute win in my book. (livinginyellow.com)