The noun [_Busen_][1] is generally a synonyme of _weibliche Brust_ (female breast).

I have only learned now, while doing research for this answer on [Wikipedia][2], 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][1] and [Duden Online][3], do not know this meaning.


  [1]: https://www.dwds.de/wb/Busen
  [2]: https://de.wikipedia.org/wiki/Busen
  [3]: https://www.duden.de/rechtschreibung/Busen