The usage of the word has changed over time.

These days, it usually means the female breast.

About 200 years ago, it was also used not for the area between the breasts, but also for the inside, the chest or rib-cage area.

This poem by Heinrich Heine contains the lines

Dem König wards heimlich im Busen bang.

This is about a king, and he starts to feel fear. It he is a man, it doesn't refer to the breasts, and also not to the area between the breasts.

The usage of the word has changed over time.

These days, it usually means the female breast.

About 200 years ago, it was also used not for the area between the breasts, but also for the inside, the chest or rib-cage area.

This poem by Heinrich Heine contains the lines

Dem König wards heimlich im Busen bang.

This is about a king, and he starts to feel fear. As he is a man, it doesn't refer to the breasts, and also not to the area between the breasts.