The noun Busen is generally a synonyme of weibliche Brust (female breast).
I have only learned now, while doing research for this answer on Wikipedia, that Busen may also be used for the depression between the two female breasts. Unfortunately, Wikipedia remains silent about the origin of this usage. Other sources, such as the DWDS and Duden Online, do not know this meaning.
Edit: Notice that the Wikipedia entry was changedchanged on June 18th 2019. The definition that Busen may also be the depression between the breasts was removedexchanged with the definition that it used to be also referring to the male breast.