A crash diet is a diet which is extreme in its nutritional deprivations, typically severely restricting calorie intake. It is meant to achieve rapid weight loss and differs from outright starvation only slightly. They are not meant to last for long periods of time, at most a few weeks.

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 flexible intermittent fasting, where dieters fast for 2 days each week and calories are cycled. Generally the weight lost in a crash diet returns when normal eating resumes.