This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. The line formed by the edge of a garment at or near the neck.

n. The line followed by an article of clothing below the neck.

n. the line formed by the edge of a garment around the neck

"The neckline is curved, and that can be done by inserting a piece of paper under your pattern and re-drawing your neckline."

neck    neck opening   

hemline   

crew neckline    décolletage    crew neck   

bodice    trendline    hem    basque    waistband    cuff    boddice    sundress