Here are answers from a variety of WikiAnswer contributors:
Generally, a fad diet is exactly what the name implies: a trendy, new diet that tends to disapear from the public view. A few years back it was Atkins, then South Beach, etc; The main problem with fad diets is that they promise (and sometimes deliver) radical weight change. Many of these diets fail to distinguish between body fat, lean muscle mass, and water weight, with the result that the dieter tends to lose a combination of all three. Why is this bad? Because muscle is metabolically active, meaning it actually burns calories. Losing a portion of your lean muscle means you will then have to eat less daily to maintain your current weight level.
A 'fad diet' is a diet that has ridiculous procedures that only have short-term weight effects and usually the weight just comes back. Its a bad diet that doesn't really work.
A fad diet is one that has a gimmick, like eating only red foods or not eating at noon.
'Fad diet' means a reducing diet that enjoys temporary popularity.
A fad diet is usually something like the Cabbage Soup Diet, where cabbage soup is the ONLY thing you eat for however many days. Other examples could be starving yourself, or eating just one or two kinds of foods or food groups per day.