Waistline refers to the horizontal line where the waist is narrowest, or to the general appearance of the waist. People who diet are often said to be trying to "improve" their waistline.
The waistline of clothing generally corresponds to the human waist.
Traditionally belts are worn around the waist.
The size of a person’s waist or waist circumference, indicates abdominal obesity. Excess abdominal fat is a risk factor for developing heart disease and other obesity related diseases. The National, Heart, Lung and Blood Institute (NHLBI)  classify the risk of obesity related diseases as high if: men have a waist circumference greater than 102 cm (40 in), and women have a waist circumference greater than 88 cm (35 in).
According to a study, women's waistlines have grown by 4.3 cm (1.7 in) in the last 10 years from an average of 80.8 cm (31.8 in) to 85.1 cm (33.5 in) inches believed to be caused by sedentary lifestyle.
Waist reduction or waist training refers to the act of wearing a corset or other constricting garment to reduce or alter the waistline. The four floating ribs may be permanently compressed or moved by such garments. A girdle may also be used to alter the appearance of the waist.
Waist reduction may be used simply to reduce the width of the waist. This change can be permanent or temporary.
Waist training may be used to achieve a certain permanent waist shape, such as a pipe-stem waist.
WAIST, the middle part of the human body, the portion lying between the ribs and the hip-bones, comprising the compressible parts of the trunk. The word is also applied to the central portion of other objects, particularly to the narrowest portion of musical instruments of the violin type and to the centre of a ship. The word appears in the M. Eng. as waste, " waste of a mannys' myddel" (Prompt. parv. c. 1440), and is developed from the O. Eng. wcestm, growth, the "waist" being the part where the growth of a man is shown and developed; cf. Icel. voxtr, stature, shape; Dan. vaext, size, growth, &c. It is thus to be derived from the 0. Eng. weaxan, to grow, wax.