cleavage:
[]

1. Informal The hollow between a woman's breasts, especially as revealed by a low neckline.
【非正式用语】 乳沟，胸槽：妇女两乳间的凹陷，尤指低领口展露的部分
the visible gap between a woman's breasts. Zero gap preferred, 1/2" is interesting, 3" is boring, 5" is too much.
(来源：英语图片 http://photo.englishcn.com)

also
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1. 劈开;开裂[U]
2. 裂开的方式[U]
3. 裂缝;分裂[C]
a sharp cleavage in society between rich and poor
贫富之间明显的分裂

(来源：英语e问e答 http://ask.englishcn.com)

  (来源：EnglishCN英语博客基地)