cleavage

The depression or valley created between a woman's breasts via partial exposure of the chest.

In anime, video games, and art, the cleavage is a popular method of adding some sexual appeal to the character. It should be noted that the cleavage is sometimes shown on characters as merely a curved line, while other times it has an extra, smaller line on the end, making the shape of a "Y" at the top of the depression.

The opposite is breasts apart.

Note: cleavage with a flat_chest is, by definition, impossible and small_breasts almost certainly need the support of a (push up) bra to have cleavage.

This tag implicates breasts (learn more).