I've heard this term bandied about on these boards, but have yet to hear a thorough and specific definition. I know Professor X is always talking about it, so maybe he--or anyone wlse with experience in this tpic--can enlighten me.
Your body is comfortable at a certain weight. It may fluctuate over the course of a day, but generally, it tries to maintain a balance. If I weighed 200lbs and over the course of two months, increased my body weight to 215lbs, returning to earlier eating habits would probably result in me losing that extra weight unless I held onto it long enough for my body to adjust to it and consider it "normal".
In fat loss, the rebound weight gain seen in so many dieters once they return to eating like normal is also evidence of this. Some fat housewife jumps on the Atkins Diet for 2 months, loses 20lbs, starts eating Ding Dongs again with extra cream filling and soon she is back to her previous bloated self. Had she maintained that weight loss for a year, there would probably be less chance of such immediate weight gain from a few dietary faux pas.
Because of this, the current trend to only eat to gain for about a month before they start dieting again seems like a wasted effort. Your body is not going to just keep gaining muscle if you never even give it time to adjust to a heavier weight. I personally have no doubt that had I not bulked up, I would be much smaller than I am now in terms of lean body mass.