The Master Cleanse is a lemon juice fast created, as far as I know, by Stanley Burroughs. You can read about it in his little book, The Master Cleanser
One thing I’d like to say is that the master cleanse is not for everybody. If you are underweight, I would not do it, unless you are under the supervision of a qualified health professional who has experience with the master cleanse. Other people who generally do not do well on it are people with what Chinese Medicine calls a cold stomach. If you have a hard time digesting cold foods, raw fruits or vegetables, or cucumbers, you have a cold stomach.
If you don’t have any of the above conditions, and you feel in need of a stronger cleanse than the one in my earlier post, then definitely check out the Master Cleanse.