Weight loss, in the context of medicine or health or physical fitness, is a reduction of the total body weight, due to a mean loss of fluid, body fat or adipose tissue and/or lean mass, namely bone mineral deposits, muscle, tendon and other connective tissue.

A crash diet is where a person willfully restricts themselves of all nourishment (except water) for more than 12 hours. The desired result is to have the body burn fat for energy with the goal of losing a significant amount of weight in a short time. Crash dieting is not the same as intermittent fasting, in which the individual periodically abstains from food. [wikipedia]