Sentence:  All this time I kept my gaze fixed on hers, an enormously difficult task given the gravitational pull exerted by her <WSD>cleavage</WSD>. While I was critical of many things when it came to so-called Western civilization, <WSD>cleavage</WSD> was not one of them. 
Meaning of the word marked with <WSD> tags:  The hollow or separation between a woman's breasts, especially as revealed by a low neckline. (from 20th c.)