I thought "der Busen" meant breasts. But then I heard someone knowledgeable claim it was just the area between the breasts. I didn't understand all he said and it would have been too awkward for me to ask him.
Here's an illustration:
Attribution: SpirosK photography from Athens, Greece [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)] Link: Aphrodite of Milos
Does "Busen" denote the area between the breasts or the breasts?
The word Busen refers to terms we all agree on
Then some you might have heard from
And some you might have never heard from
Just to name a few.
All of these definitions can be found in the Deutsches Wörterbuch von Jacob Grimm und Wilhelm Grimm. But it says nothing about 5. the area between the (female) breasts. Can we say Busen doesn't denote the area between the breasts?
Let's take a deeper look.
One way is to look for historic examples where someone refers to the area between the breasts as Busen.
[...] und gab es dem Weib, so wie ich auch die übrigen [Eier], die ich sorgfältiger geöffnet hatte, meinen übrigen Mägden gab, sie in ihren Busen zwischen den Brüsten bis zur Zeit des Ausschliesens zu erwärmen.
source: Der Römisch-Kaiserlichen Akademie der Naturforscher auserlesene medizinisch-chirurgisch-anatomisch-chymisch- und botanische Abhandlungen, Band 20 from 1771
Will it be so easy? Unfortunately I don't think so. I'm pretty sure the author refers to 6. a fold in the upper part of your clothing, where he told his maiden to put the eggs, so they are warm and can hatch.
Ok next, at least there is one thing we can say so far: Anatomists do say Busen, when they are refering to the area between the female breasts, as anatomists Helmut Wicht is quoted in this blog entry and there are a lot of other online dictionaries, where we can find this.
Anatomists call the area between the female breasts
Sinus mammarum (old)
Sinus intermammarius
Sulcus intermammarius
or sometimes just short
Sinus
Sulcus
where Sinus can be translated as
Bauch, Tasche, Bucht (paunche/bulge, pocket, bay)
and Sulcus as
Rille (groove)
But OMG! Wait a minute and let's go back to Grimm's Wörterbuch. Right there ... in the very first line
busen, m. sinus, κόλπος, sl. nadro, njedra, ahd. puosum (Graff 3, 218)
Is Busen for the area between the female breasts no more than a direct translation of the latin word Sinus, which is short for Sinus intermammarius/mammarum the anatomical name for the area between the female breasts? It makes sense.
Also, Helmut Wicht assumes, that it all started with the book Die alten deutschen Kunstwörter der Anatomie, because it's the root for a lot of Eindeutschungen. But I'm not sure about it, but take a look for yourself. But if it is true and the new meaning established around 1890 it wouldn't be found in the (earlier) Grimm's Wörterbuch.
I think it's perfectly fine to say that the word Busen is referring to the area between the female breasts and no one will doubt it refers also to the female breasts. And even if I'm wrong with this derivation, anatomists are saying it anyway.
Fun fact: According to Grimm's DWB when talking about breasts Busen always refers to both breasts and the plural Busen refers to the breasts of multiple women.
According to Wikipedia it can mean both things.
or
But especially in daily use I would assume most people use it as the first definition. German is my mothertongue and I honestly did not know about the second definition.
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.
The word is, as far as I can see, almost completely equivalent with the English bosom. Both come from a West-Germanic root. Both can mean female breasts, "especially as a single feature", as Merriam-Webster succinctly puts it ("sie hat einen schönen Busen", meaning both breasts), as well as male or female chest.
The contemporary meaning in German is, in my experience, shifting towards being a synonym for the female breasts. The old usage (e.g. a male speaker exclaiming to an old friend "Komm an meinen Busen!") sounds funny to me and would likely be used as an ironic exaggeration only.
As always with old words there is a certain gap between poetic, formal or generally high-level language on one side and casual everyday use on the other. In casual language I think the word is used exclusively as a synonym for "female breast", even in separation. For example it would not sound immediately wrong to me (as it did for the Grimm brothers as quoted in Michael Hoppe's answer) if one said "ihr linker Busen ist größer als ihr rechter".
Answering and discussing the question I learned that indeed in anatomy the gap between (probably female) breasts can be called Busen, in a translation of sinus [mammarum] (see e.g. an entry in the Brockhaus). The background is perhaps that the sinus in the abstract sense is a curve which can go both ways; Meerbusen also is a translation of sinus, sinus maritimus.
I initially went and removed the "gap" meaning from the wikipedia page which initiated this discussion; but as I said I found several dictionary entries supporting that meaning in an anatomical context and now re-added the "cleavage* meaning on the wikipedia page. All edits are still pending. Sorry about all the work for the reviewers, whoever they are.
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 changed on June 18th 2019. The definition that Busen may also be the depression between the breasts was exchanged with the definition that it used to be also referring to the male breast.
A word where you can still see "Busen" referring to something with a concave shape is "Meerbusen" (= "sea bosom") which just means a big bay.
"Busen" and "Brust" are quite different: "der weibliche busen, doch nicht im sinne von mamma, uber; man kann nicht sagen die milch im busen, dem kind den busen geben statt die brust; gleich unstatthaft wäre der rechte oder linke busen für die rechte oder linke brust."
from the Grimm Brothers, see http://woerterbuchnetz.de/cgi-bin/WBNetz/wbgui_py?sigle=DWB&mode=Vernetzung&hitlist=&patternlist=&lemid=GB13460#XGB13460
NB: "Heutzutage" wird mit "Busen" meist Anderes gemeint. Als ich als Bub Märchen las, steckte sich oftmals jemand etwas in den Busen -- ich konnte mir beim Willen nicht vorstellen, wie das halten sollte ... Bis ich eben vor einigen Tagen im Grimm unter (3) las: "das die (weibliche oder männliche) brust zwischen armen und hüften hüllende, sich darum biegende gewand". Endlich Klarheit nach fünfzig Jahren!
"Der Busen" (bosom) is generally the female "Brust" (breast) but in the whole, meaning both breasts. (Wikipedia)
Duden differentiates it from the female breast as following:
Female breast in its vivid appearance especially in regards to its erotic appeal.
Duden also indicates it as a synonym to the female breast or breasts but more in a poetic or antiquated way.
As a native speaker i would say, it simply means the whole female "breast area" but clearly not just the area between the breasts.
On the other hand, Wikipedia states in its article about "breast" that the area between two breasts is called "Busen". This is also describes in this article (in German).
But how come that this meaning is so unknown under German native speakers? There is an interesting article about this question (sorry, again German). Summary: It is the former or rather historical meaning.
The correct (or rather modern) name for the area between breaths is "Sulcus intermammarius" btw.
Does "Busen" denote the area between the breasts or the breasts?
As the question in a paradox reading implies (area between (the breasts or the breasts)) the answer is yes--which wouldn't happen with singular "the breast".
Whether the ridge gave a meronym, a subsense, of the more general whole, or vice-versa, is anyhow not part of the question, but still interesting to ask. I'm afraid I cannot decide it, and the thread so far is not completely clear either. The majority seems to think the word had always described tits.
Indeed, -en in most cases shows plural. der Busen would thus be a pluraletantum like die Eltern, and this point is driven further by the lack of a separate plural morpheme in die Busen (which would thus imply two peoples'bosoms). This is just a hint, not compelling evidence, as the English and German forms differ too much, which is expected for ever so slight taboo words. Further considerations would have little to do with modern German, so I will skip analogous comparisons of tit and zit.
The question can be parsed as an inclusive or. In that sense, the chest bone area as the center of gravity of the bosom is the anatomical human chest more truly than the "milk bags" that bosom today generally refers to.
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