Sheath refers to a straight, body-hugging, narrow fitted dress usually with no waistline but shaped to the body by vertical darts or with a set-in waistline.
Sheath refers to a short, slim-fitting dress that is cut to cinch in at the waist without a belt or waistband.
Other /More definition:
Sheath refers to a straight dress with no waistline. (See Sheath Dress)