Obesity is a condition in which excess body fat has accumulated to such an extent that health may be negatively affected. It is commonly defined as a body mass index (BMI = weight divided by height squared) of 30 kg/m² or higher. This distinguishes it from being overweight as defined by a BMI of between 25–29.9 kg/m².

Excessive body weight is associated with various diseases, particularly cardiovascular diseases, diabetes mellitus type 2, obstructive sleep apnea, certain types of cancer, and osteoarthritis. As a result, obesity has been found to reduce life expectancy. The primary treatment for obesity is dieting and physical exercise. If this fails, anti-obesity drugs and (in severe cases) bariatric surgery can be tried.

Obesity arises from too much energy intake compared with a person's basal metabolic rate and level of physical exercise. Excessive caloric intake and a lack of physical activity in genetically susceptible individuals is thought to explain most cases of obesity, with purely genetic, medical, or psychiatric illness contributing to only a limited number of cases. With rates of adult and childhood obesity increasing, authorities view it as a serious public health problem.

Although obesity is often stigmatized in the modern Western world, it has been perceived as a symbol of wealth and fertility at other times in history.

Classification

Obesity, in absolute terms, is an increase of body adipose tissue (fat tissue) mass. In a practical setting it is difficult to determine this directly. Therefore obesity is typically assessed by BMI (body mass index) and in terms of its distribution via the waist circumference. In addition, the presence of obesity needs to be evaluated in the context of other risk factors such as medical conditions that could influence the risk of complications.

BMI

Body mass index or BMI is a simple and widely used method for estimating body fat mass. BMI was developed in the 19th century by the Belgian statistician and anthropometrist Adolphe Quetelet. BMI is an accurate reflection of body fat percentage in the majority of the adult population. It is less accurate in people such as body builders and pregnant women in whom body composition is affected.

BMI is calculated by dividing the subject's weight by the square of his or her height, typically expressed either in metric or US "Customary" units:

Metric: $BMI\; =\; kg/m^2$

where $kg$ is the subject's weight in kilograms and $m$ is the subject's height in metres.