This is a vintage 60s Siren nylon nightgown. Made of sheer outer layer of peach or apricot nylon over a more solid nylon tricot lining, it has embroidery at the center front bust beneath a round neck with lace trim. Sleeveless.
 
Pre-owned, used; in good condition overall BUT the sheer outer layer is has shrunk and is shorter than the under layer AND the hem on the lining layer is partially unstitched. I'll mend the unstitched hem but can't fix the different lengths of outer and inner layers.
 
Marked as a size S. The actual garment measurements are given next--
 
Bust- approx. 38" measured around at the underarm seams
Pit to pit- 19" across
Front shoulder to hem length- 34 1/2"
Hem- 48" around