What exactly is crash dieting?

Severe low calorie. I guess is a sudden dramatic cut on food intake for short period of time with a goal of losing too much in a short period of time. It is considered dangerouse.
Crash dieting. Crash dieting is going on an extreme diet. By that I mean not just eating fewer carbs or sweets, but extreme as in a liquid diet, or starving yourself, or both plus taking diet pills. Not healthy. Not recommended. Never successful for long term weight loss.