The waistline refers to the horizontal line where the waist is narrowest, or to the general appearance of the waist.
Definition: "middle part of the body," also "part of a garment fitted for the waist, portion of a garment that covers the waist" (but, due to fashion styles, often above or below it), probably from Old English *wæst "growth," hence, "where the body grows," from Proto-Germanic *wahs-tu- (cognates: Old English wæstm, Old Norse vöxtr, Swedish växt, Old High German wahst "growth, increase," Gothic wahstus "stature," Old English weaxan "to grow". 
In modern clothing the region referred to as the waist is considerably below the waist as defined anatomically. With the advent of pants and skirts that do not require support from above, the clothing waist moved down to a position where the body starts to expand to form the buttocks and a support is therefore available. The waistline of clothing is now generally well below the anatomical waist.
Because of this and because the waist is often synonymous with the stomach, one can become confused as to the exact location of the waist. Another confusing factor is that the waistline differs on different people. To locate the natural waistline, one need simply stand upright and then tilt over to the side keeping the legs and hips straight. Where the torso creases is the natural waistline. late 14c.,
Waist–hip ratios of 0.7 for women and 0.9 for men have been shown to correlate strongly with general health and fertility.
Strictly, the waist circumference is measured at a level midway between the lowest rib and the iliac crest. The waist-hip ratio equals the waist circumference divided by the hip circumference. Practically, however, the waist is usually measured at the smallest circumference of the natural waist, usually just above the belly button. In case the waist is convex rather than concave, such as is the case in pregnancy and obesity, the waist may be measured at a vertical level 1 inch above the navel.
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)  classifies 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).
Waist reduction and training
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.
- Belly chain
- Navel piercing
- Waist-hip ratio
- Human physical appearance
- Body modification
- Abdominal obesity
- Rib removal
-  Han, T.; Van Leer, E.; Seidell, J.; Lean, M. (1995). "Waist circumference action levels in the identification of cardiovascular risk factors: prevalence study in a random sample". BMJ (Clinical research ed.) 311 (7017): 1401–1405. doi:10.1136/bmj.311.7017.1401. PMC 2544423. PMID 8520275.
- Waist To Hip Calculator at University of Maryland Medical System. Retrieved Dec 2010
-  Brown JE, Potter JD, Jacobs DR, et al. (January 1996). "Maternal waist-to-hip ratio as a predictor of newborn size: Results of the Diana Project". Epidemiology 7 (1): 62–6. doi:10.1097/00001648-199601000-00011. PMID 8664403.
- "Obesity Education Initiative, Aim for a Healthy Weight".
- Lawrence de Koning; Merchant, AT; Pogue, J; Anand, SS (2007). "Waist circumference and waist-to-hip ratio as predictors of cardiovascular events: meta-regression analysis of prospective studies". European Heart Journal 28 (7): 850–6. doi:10.1093/eurheartj/ehm026. PMID 17403720.