Fat is a lay non medical term. Obesity is defined as a BMI > 30 in adults
What are overweight and obesity?
Overweight and obesity are defined as abnormal or excessive fat accumulation that may impair health.
Body mass index (BMI) is a simple index of weight-for-height that is commonly used to classify overweight and obesity in adults. It is defined as a person's weight in kilograms divided by the square of his height in meters (kg/m2).
Adults
For adults, WHO defines overweight and obesity as follows:
overweight is a BMI greater than or equal to 25; and
obesity is a BMI greater than or equal to 30.
http://www.who.int/mediacentre/factsheets/fs311/en/
Health Risks
Are related to the degree of being obese.
It makes you more likely to have conditions including:
- Heart disease and stroke.
- High blood pressure.
- Diabetes.
- Some cancers. Increased rate of relapse from eg. breast cancer
- Gallbladder disease and gallstones.
- Osteoarthritis.
- Gout.
- Breathing problems, such as sleep apnea (when a person stops breathing for short episodes during sleep) and asthma.
- Psoriasis
- Poor response to drugs for arthritis
https://www.webmd.com/diet/obesity/obesity-health-risks