Indeed, it is not a taboo subject anymore! In fact, I frequently read about it in news papers mentioning this as dressing sense or beauty of a woman (I guess there are competitions as well).
Whatever–a dressing sense–or naturally forming 'deep cut', some have it well-defined, others simply show its absence.
Now there could be two opposites in this case and I am searching for both those terms.
A) What's the opposite term for the word 'cleavage' – a woman with absence of cleavage (flat or no cleavage does not convince me).
AND…
B) What's the term that describes opposite of cleavage itself – cleavage is *the space between the organs that can be seen above a dress but then some dresses (not uploading a picture here to keep this site graphically clean) show them from the bottom i.e. below the top (opposite direction)? What's the term for that? Cleavage from Bottom? 'Bottom Cleft' – no way. No word with bottom (#7) can prevent ambiguity here.
Best Answer
"A woman lacking cleavage" is not the opposite of "cleavage," it's the opposite of "a woman showing cleavage," which there is not a single word for, either.
Not every word--even a word that is "a feature of something"--has a single word you can point to as its "opposite." What is the opposite of "redheaded"? It's "not redheaded." Similarly, the opposite of "showing cleavage" is "not showing cleavage."
Cleavage, as it's used in American English at least, is also not an inherent feature of a woman's body; it's an effect produced in large part by the way the woman is dressed. It doesn't matter how a person's body is shaped; if she is wearing a turtleneck sweater, she is not going to show any "cleavage."
As for cleavage shown at the bottom of the breasts, impeccable source Jezebel calls it "underboob."