Obesity is a medical term which describes a condition where excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems.
One definition of "fat" is a layman's term for obesity.
The terms fat or thin should be no more derogatory than tall or short, having brown hair or blonde hair. They are just descriptions which should be neutral regarding desirability. Yet we are brainwashed into believing some body types are more desirable than others. Desirability is a personal preference, not a universal standard.
Perceived desirability is influenced by cultural and personal tastes. Many cultures and individuals find obesity attractive or desirable. Obesity can be perceived as a sign of wealth and affluence. However in western culture, we've become obsessed with being thin... in the case of super models, the current standard is near anorexic and unhealthy IMO. Personally, I find the body shape of most super models unattractive. They are too thin according to my personal tastes. I find women with a curvy solid athletic build desirable like these women athletes:
To me, the above women have a far more desirable body shape than any super model. They are fit and healthy. A women wearing khakis and hiking boots is far more attractive to me than a women wearing a ridiculously uncomfortable dress, nails, makeup and stilettos.
I don't expect other people to share my personal tastes, nor do I expect people to meet my personal perception of attractiveness. People who are comfortable with their bodies are more attractive to me than people who aren't. Personality and character are more important compatibility requirements for me than appearances.
For the record, I am 5'8" and about 190lbs. I'm overweight, but in decent physical condition. I have been as heavy as 210. I would describe myself at 210 as obese. I'd like to be 170 lbs, not to meet someone's expectations, but to meet my own expectation regarding health and fitness. I'm not about to get bent out of shape if someone describes me as fat or obese, because I don't have body image problems like some people here obviously do. I'm just grateful I am still healthy.