i Register
In some senses, waistline is marked as slang. Watch for register when choosing this word.
noun
a line around the body at the waist; its measurement
the narrowest part of a garment, usually at the waist, but may be above or below depending on the dictates of fashion or the whim of the designer
person having a large waistline
The horizontal line just below the windows of a car or other vehicle.
[...] with the adoption of a curved bodyside profile Bulleid considerably raised the height of the waistline and at the shoulder height of seated passengers, just where the extra space was most needed.