Earlier during the prehistoric period, food supply was very irregular, so our body developed the ability to store excess food for the immediate requirement of survival. It is stored in the form of fat, that accumulate in the adipose tissue. Adipose tissue is connective tissue consisting mainly of fat cells.
Adipose tissues have a key role during starvation or fasting. At time of fasting, the energy is liberated by the breakdown of fat that is stored in the adipose tissues. However with abundance of nutritional food intake and sedentary life style, increased the fat reserves of adipose tissues leading to obesity that adversely affect the health of the individual.  
Definition:
Obesity is the abnormal accumulation of excessive amount of fat which may impair the health of the individual.
It is described in terms of Body Mass Index  or BMI. BMI is the person's weight in kg divided by square of his height in meter. As per WHO, An individual is said to be obese when the BMI is greater than or equal to 30. And if the BMI greater than or equal to 25 it is said as overweight.There are other method of quantifying obesity including anthropometry, densitometry, CT or MRI, and electrical impedance.
Incidence:
Worldwide obesity has nearly doubled since 1980.
Around 3.4 million adults die each year as a result of being overweight or obese.
65% of the world's population live in countries where overweight and obesity kills more people than underweight.
Causes and Risk factors:
The fundamental cause of obesity the energy imbalance between calories consumed and calories utilized.
Risk factors include
1. Heredity
2. Gender
3. High dietary intake of fatty foods
4.Food rich in calories
5. Lack of adequate physical activity and exercise
6. Sedentary lifestyle
7. Underlying medical illness like hypothyroidism.
Health hazards of obesity:
Obesity leads to many health consequences. It is a vital risk factor for most of the non communicable and lifestyle diseases.
Obesity causes cardiovascular diseases, stroke, hypertension, diabetes, musculo-skeletal disorders (especially osteoarthritis - a highly disabling degenerative disease of the joints), and even some cancers (endometrial, breast, and colon).
Obesity is preventable.
1. Obesity and the associated non communicable and lifestyle diseases are highly preventable.
2. Limit the dietary intake of fatty foods.
3. Avoid Jung foods.
4. Increase consumption of fruits and vegetables, whole grains etc.
5. Restrict amount of sugar.
6. Do regular exercises (60 minutes a day for children and 150 minutes per week for adults).
7. Periodic health check-ups.