Crash diets refer to extreme weight loss programs that promise significant results in a short period, typically within days or weeks. These diets often involve severe calorie restriction and elimination of entire food groups, leading to rapid weight loss but also posing health risks such as nutrient deficiencies, dehydration, and metabolic changes that can slow down the body's ability to burn fat in the long run. Crash diets are not recommended for sustainable or healthy weight loss, and it is better to adopt a balanced diet and regular exercise as part of a lifestyle change for maintaining a healthy weight over time.