The space between a woman's breasts is called cleavage.
Cleavage refers to the narrow depression or hollow that naturally occurs between a woman's breasts. This area can be more or less visible depending on factors such as breast size, shape, and the type of clothing worn.
In many instances, clothing like low-cut tops is specifically designed to accentuate the upper portion of the cleavage. While the term "décolletage" technically refers to the low neckline itself, the term "cleavage" is often used interchangeably to describe both the visible space between the breasts and the revealing neckline that displays it.
