Dear Washington Post,
Since you seem to be unclear on the concept, I would like to present to you some examples of actual cleavage (Figs. 1-3), with markings helpfully provided so that it'll jump right out at you, so to speak.
Fig. (1): Joanna Krupa
Fig. (2): Catherine Bell
Fig. (3): Elvira, Mistress of the Dark
In each of those three figures, there is a clearly defined line or "valley" between the sweater puppies, suitable for holding a shot glass or dollar bills or what have you. In Figs. (2) and (3), the exposure of said line is great enough that it borders on the elusive "sideboob" [q.v.].
In the example you have cited (Fig. 4), no such line or valley exists.
Fig. (4): Sen. Hillary Clinton
Therefore, while Sen. Clinton's shirt may, by some conservative definitions of the term, qualify as "low-cut," there is no "cleavage" on display here.
In the future, should you find yourselves confronted by semiotic questions of this nature, feel free to consult with me; I'm kind of an expert on these things. Or, alternatively, you could treat the presidential race with the respect that it deserves and not like a goddamn high-school prom, and thus refrain from covering non-stories like this at all.
Yeah, I'd probably go with that last one.
Founder and CEO, Hey Jenny Slater Enterprises p.l.c.
Lucy K. Pinder Endowed Chair of Mammary Arts and Sciences, Birmingham Institute of Lechery
(dictated but not read)