Definition: a dress with a low neckline; the area of the body exposed by such a dress
There’s a good etymology here: it comes from the French word décollet, which literally means no (“de”) collar (“collet”) to represent “low neckline.”
In one type of usage, décolletage is also known, more simply, as cleavage.
Why I Looked It Up
From a book about the Vanderbilts:
[Mrs. Astor’s] ears sparkled with diamond drop earring, her chest glittered with a diamond bow and lovers’ knot brooches, and her décolletage was weighted down with heaps of diamond necklaces.