das Dekolleté
neckline
A low neckline on a women's garment that leaves the neck, shoulders, and upper part of the chest uncovered.
Das Abendkleid hatte ein sehr elegantes Dekolleté.
The evening dress had a very elegant neckline.

décolleté / neckline
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neckline
A low neckline on a women's garment that leaves the neck, shoulders, and upper part of the chest uncovered.
Das Abendkleid hatte ein sehr elegantes Dekolleté.
The evening dress had a very elegant neckline.

The meaning depends on the context. If people are talking about a garment ('das Kleid hat...' — the dress has...), it means the neckline. If they are talking about the body ('Haut' — skin, 'eincremen' — to apply cream), it refers to the body area.
Das Brautkleid hatte ein wunderschönes Dekolleté aus Spitze.
The wedding dress had a beautiful lace neckline.
When 'Dekolleté' (neckline) describes the cut of a garment, it is often combined with adjectives such as 'tief' (low/deep), 'weit' (wide), 'elegant' (elegant), or 'herzförmig' (heart-shaped).
Für den Gala-Abend wählte sie ein Kleid mit einem tiefen Dekolleté.
For the gala evening, she chose a dress with a low neckline.
When the body area is meant, the word is used like other body parts. For example, you can wear jewelry 'auf dem Dekolleté' (on the décolleté) or care for the skin 'am Dekolleté' (on the décolleté area).
Die Kette lag perfekt auf ihrem Dekolleté.
The necklace lay perfectly on her décolleté.
chest / breast
'Brust' and 'Dekolleté' both refer to the upper area of the body. 'Brust' is the general word for the body part. 'Dekolleté' means either a low neckline on a garment or the visible upper part of the female chest created by it. The words can be confused because both touch on the same area of the body. When you mean the body part in general, you say 'Brust'. When clothing, a neckline, or the visible area is meant, 'Dekolleté' fits better.

Das Kleid hat ein schlichtes Dekolleté und wirkt sehr elegant.
The dress has a simple neckline and looks very elegant.
Die Ärztin hört die Brust des Patienten ab.
The doctor listens to the patient's chest.